Been having some ISaT tech level and timeline (as in 'when backstory things happened') thoughts and want to ramble a bit:
Tech
ISaT's setting is based on your classic sword and sorcery fantasy JRPG so its easy to assume that tech wise everything is 'fantasy medieval' and call it a day but consider:
Body Craft is at bare minimum is magic sex changing surgery (earliest known equivent in our world dating to 1930) that any old person can learn to do safely on themselves in only a few months, and more likely has many many broader applications like regular old surgery, the 'combat healing' Mira and Sif use, etc.
Cameras and photos are rare enough that Odile comments on it (namely she's happy the group get a photo in the House due to them being 'so rare') yet are also common enough that everyone in the party knows about them and doesn't question the existence of a camera beyond being surprised that the mirror was one. (In our world cameras date back to the 19th century, with the earliest manufacturing of them being in 1839).
Printing presses and ways to make plenty of paper to feed those presses given absolutely everyone in this game can read, expects everyone else to know how to, and both mass produced book series and newspapers are a thing. (Note: Printing presses have existed since waaaay back but it's the mass paper manufacturing that makes newspapers and The Cursing of Chateu Castle possible that really has my eyebrows raised here, especially since neither Siffrin or Odile find either odd in any way, indicating such things are common everywhere, and while newspapers have been around since 1604 in our world, mass produced fantasy books didn't really take off until the start of the 20th century).
Food production and storage: despite being in the middle of a national disaster that almost certainly cut off trade networks and access to most suppliers for literal months now, Bonnie, a small child, is able to easily get their hands on fresh Pineapple, curry ingredients (for samosa), potatoes, plantains etc with no issues or anyone commenting on this being unusual or lucky. Oh and the only character who even brings up the concept of potential starvation is the Fishing One, and only in a sort of 'we're not at risk now but sooner or later...' kinda way due to noticing that the fish they fish up for fun are disappearing and likely being frozen. So yeah, that heavily implies Vaugarde has very good food storage tech/Craft (possibly better than ours), and likely also good food production and harvesting tech/skills also.
The Island's incredible knowledge of the stars: while the oldest known orrery in our world is dated from around 205 to 87 BC (ancient Greek, earth centric model), the fact Sif -who would've learnt this as a child/teen- is so very certain that stars are big balls of fire made up of gas is interesting as that's something our world couldn't prove the theory of until around the 1900s (note: it'd been theorized a LONG time but Sif talks like its complete facts to them . Additionally the earliest existing record of a telescope in our world dates to a 1608 patent and we see one of those in game.
Post posting EDIT: A wonderful user qds-place pointed out that Mirabelle has anti-anxiety medication in her room. This is in both ISaT and SAaP and though we're not sure what form the medication takes (pills? Valium? Megitech esc Craft boosters???) the fact they specifically have 'anti anxiety' medication at all (as opposed to idk dragging Mira off and drugging her through the gills) is kind of impressive and if it IS modern anti anxiety pills those could be as recent in creation as the 1950s! So um. Some high levels of tech implied in chemistry there <3
So... yeah. All this, plus the fact that the highest tech implied area, The Island, literally specialised in the study/Craft of turning wishes into reality (for a long enough time period that Wish Craft is culturally so ingrained in the King and Sif that they do it without thinking and it seems intertwined with their nation's religion) has been completely erased from memory to the point anything heavily associated with them has been forgotten, and also we only ever see Dormont aka a little village well away from the cities, it's not hard to conclude that you can basically justify giving the ISaT world any level of tech you want, so long as you lock any of the truly 'setting breaking' stuff like planes and rockets behind The Island's forgetting curse (I would've said trains too but thinking about it trains were invented in 1802 so it's honestly easy to imagine that they totally exist in the ISaT setting/Vaugarde but aren't ever on screen because rail is way too dangerous to consider using while the Curse is active and potentially time freezing things on the tracks).
Side note: We know absolutely nothing about Vaugarde's transport system but as a fan of fantasy RPGs it is honestly a travesty I have yet to see a fic that has flying dragons/wyverns or other fantasy mounts in setting. Like, ok yes, the party would probably have wanted to use those but maybe they don't like the Curse and fled? Maybe the King's Curse targeted them first? Maybe all their handlers dropped the heroes off in Dormont and said 'Well Saviors it's been fun, but well me and Scales here are off to Poteria until things wrap up so best of luck to you' before buggering off?
This isn't really a serious complaint just. Me reminding myself/potentially other fanwork writers out there that there's a lot about the setting we just don't know about and limiting all travel to walking, horse drawn carriage and boat is not actually required. (Also please mix up travelling to the Island. Boats are a wonderful classic and have great thematic vibes for Sif's original leaving of the Island but like. Imagine the sheer in-universe wtf of the memories of The Island suddenly coming back and people on the north coast suddenly realising there's a massive bridge, subway or underwater tunnel leading there that everyone just forgot about - potentially filled with all kinds of Sadnesses that need taking down. Or Warp Panels in a House of Change, idk XD).
Timeline
Canon notes first:
Bonnie is a preteen (8 to 12), Mira and Isa are in their early to mid 20s (with Isa slightly older), Sif is late 20s to 30, and Odile is 40+ Nille is stated to be around 18 to 20.
Siffrin ran away from home when they were a teenager (13 to 17? 18?) and this is heavily implied to be when the Island was Forgotten.
Bonnie (in ISaT specifically*) says that Nille told them that when it happened all the adults were talking about it, hence why they think The Island is close to their village. *In Start Again a Prologue, Bonnie says that they themself remember the adults talking about the Island disappearing, which er. Is a bit impossible given they likely weren't even born yet when that happened but that can be explained away by AU differences, InsertDisc5 still finalising details between SAaP and ISaT, and/or OG Siffrin having been in the loops so long they weren't actually listening when Bonnie was talking and just 'scripted' in their head something 'close enough' to what Bonnie was saying to get the idea (note: mentioned that idea before in my post here on the differences between the House and King in Start Again vs In Stars and Time for anyone curious so er please feel free to give that a read if you haven't already).
Odile mentions remembering 'when it happened' as well and has been 'travelling for years'.
The King 'appeared out of nowhere' sometime in his adulthood, and lived in the city of Corbeaux for a few years before he became the King.
The King became the King as was freezing people in time long enough before his attack on the House of Dormont that everyone inside knew he was coming, there were a wall's worth of newspaper articles about him, and everyone was expecting Euphrasie to defeat him.
Mirabelle's quest began 'almost a year ago' and Sif lost their eye 'recently'.
Thoughts on the above:
Calculating when The Island was forgotten:
Sif being mid 20s to 30 and having run away from home as a teen means that The Island has to have been forgotten somewhere between 9 to 17 years ago with nine only possible if he ran away at age 17 and is only age 26 now, and seventeen being the far opposite if he ran at age 13 and is currently 30.
To narrow down the timeline: Given Sif ran away from home because he 'didn't want to eat his veggies' and 'just wanted to scare [his] parents a little bit' it's probably safe to assume Siffrin was likely on the younger end of the teen spectrum (teens run off all the time sure but with loving parents and over veggies? That screams 'kid who has not yet learned that freaking out the parents will get their ass grounded and/or yelled at a LOT and is therefore best saved for doing fun forbidden stuff that ideally the parents will never find out about' XD) Additionally given Siffrin can't remember his age/birthday etc but Isabeau outright says near the beginning of the game "But you're older than most of the people here?" meaning Sif must be visibly older than Isa or Mira, so he's probably closer to 30 than not.
Those alone would imply the Island likely disappeared closer to the '17 years ago' side of things BUT Nille (tops 20 years old) told Bonnie that "[the Island's disappearance] was all the adults would talk about for ages" and kids usually can't remember anything prior to 4 years of age so with that in mind...
I'd say The Island most likely disappeared between 13 to 16 years ago.
Nille stuff:
This is more a general mention but. Nille is tops 20 years old. Bonnie is between 8 and 12 and doesn't remember their parents at all.
This means Nille ran away with Bonnie and gained emancipation and custody of Bonnie (if Vaugarde has formalised that kind of legal stuff) while she was at most 12 years old herself and could have in theory been as young as 6..!
Regardless, it's very likely the original home situation was that bad, Nille deserves a ton of credit for raising Bonnie as well as she has and I'd say it's very VERY likely she had a lot of help from villagers in Bambosche and/or the local House of Change in doing so. ...But also Bonnie is very adamantly 'my sister and village' and not 'my sister and [specific names who live with us]' so there's clearly by the time Bonnie was 4 or so they were living in their own place so... Yeah. Lotta drive for independence there too it seems (so the party might have more trouble adopting Nille into their group post ISaT than Bonnie might expect).
King stuff:
Already an adult 13 to 16 years ago so at bare minimum 33. Given his vibe probably much older though.
Newspapers get printed pretty quick though for there to be so much speculation and research done into his background so quick, either Vaugarde has some form of fast messaging system (something like a Chappe telegraph on top of the Houses of Change? Odile I think does mention that they'll have a message sent to let Nille know they'll be returning Bonnie...) or the King was freezing stuff for IDK around a month or two before reaching Dormont? Alas can't find out how long it takes to walk across all of France out very easily (I'm sure the numbers are out there but my brain is pudding rn) but if we had those numbers we could probably make some guesstimates based off the rough sketched map of Vaugarde InsertDisk5 did... Which I would link but apparently the tumblr post I had it linked on has been deleted???? 'wails at this very unhappy development'
Mira's journey and Sif's eye:
We really don't know a lot but almost a year ago gives us somewhere around 9 to 11 months to spread the journey out along and after eye removal surgery the patient can out and about as soon as 2 to 6 weeks after, maybe sooner with magic healing (though full recovery/growing used to the changed spacial awareness -which Sif clearly does not have- probably can't be sped up and takes around 3 to 6 months) so um. I'd guestimate Sif's eye injury is really recent; like two months ago tops recent. ...Which sorta explains a lot of why Bonnie is not dealing with it right now and also why the others might be trying to avoid bringing it up (since Sif clearly loves avoiding the issue but they haven't yet realised that maybe they really should bring it up even if it annoys them anyway?)
Odile with some Ka Bue speculation:
When it comes to The Island, how did Odile, presumably living in Ka Bue at the time, remember 'when it happened'? Was the Island well known enough even on the other side of the world that it's disappearance made waves? Or was Odile herself or someone she's close to paying attention to the region? (Like maybe her dad or a friend is/was into politics or trade, keeping up with overseas news and got concerned it could happen to Ka Bue? I'd say 'I remember when it happened' line implies it was more immediate knowledge than being informed by a messenger much later though...)
As for Odile's 'years of travel' I have to wonder, what's left behind for her in Ka Bue? She brings up going back there quite a bit, might just miss home and possibly her father if he's still alive, but given it took her years to get here for something so personal rather than idk 'materially rewarding' I think Odile might have some kinda family estate or something back in Ka Bue... Something she wasn't worried about potentially losing while far away, but solid enough to want to return to, beyond her father who she'd definitely want to see again if he's still around. (...But given how open she is to chilling about Vaugarde a few more months with the others, I really don't think he is alive, since well, given their respective ages and travel between Vaugarde and Ka Bue apparently taking years, there'd definitely an uncomfortably high chance of him passing away while she's gone and that seems like the thing that'd stress Odile out so... Yeah. Probably got an estate in Ka Bue she'd like to take the Family to visit/possibly sell off if she decides she'd like to live with them in Vaugarde so... Just my off the cuff headcanoning here and hoping that gives others ideas or something).
---
Anyway that's all the ramble I've got in me so... yeah! Hope this was interesting and useful for those needing a bit of a 'possible tech'/timeline calcs breakdown for the Island + a few more vague things and um. Probably will post a long winding ramble about my attempt at a ISaT Selkie AU fic I've been working on next <3 (Not to be confused with looped-140-and-counting's already existing and quite wonderful Selkie Siffrin AU which already has a completed oneshot fic, a snippet of sequel, two snippets of prequel/Sif flashbacking and I believe a comic too, all of which I highly recommend <3)