occasionally I see people go like "man you could get away with saying some wild shit back in 2016 undertale but barely any headcanons in current deltarune" and that's definitely true but I think the reason's a bit deeper than just 'times have changed'. Instead, I find it lies in the fact that old undertale and current deltarune were fundamentally different kinds of fandoms.
2016 undertale was what I call an 'AU-centric' fandom, which was why there were so many AUs. What this also meant was that each individual post or fanwork was compartmentalised into its own space, which allowed for a) canon to be pretty much obsolete as people went wild and made what they wanted, and b) a hell of a lot of contradictory works/headcanons/ideas.
On the other hand, current utdr (especially deltarune), is something closer to a theory-centric fandom. This means that a majority of fanworks are considerably more grounded in reasonable canon as opposed to the more buck-wild style of classic undertale, as well as probably the worst drawback of the current fandom, being that people often attribute any work as something indicative of the creator's personal theory or headcanon about the game. This leads to a lot more dogfighting within the community as non-canon-compliant works are sometimes thought to be genuine beliefs about a character/theme; this isnt to say that nobody personally headcanons the same things as the work they make, but I think its important to remember the fact that headcanons can be directly contradictory to canon, and also to remember our roots in that things dont have to be exactly as canon is so long as a claim about the canon character isnt overtly made.
[That last part is basically just a rambling about older, more gendered depictions of frisk and chara, and how perspective on those depictions has evolved from simply being self-contained fanart into enby-erasure, an evolution in perspective which i personally disagree with.]
Anyways, thanks for coming to my shitty ted-talk yall. I made this at 2:48 am so its pretty poorly formulated. Might make a longer, slightly more clear version at a later date. Don't count on it.