I disagree with anon on most counts but I do low-key agree on some points so I would like to offer my thoughts. Apologies in advance for how long this will get.
First and foremost, these characters are indeed all privileged and rather classist, but their characterisations beyond that are not monolith.
The example of Henry insulting Simon in the classroom debate is very valid, but that does not come out of thin air. It's a response to Simon criticising the rich (i.e. the families of everyone present) and directly confronting Henry about the massive agricultural subsidies that his father gets for the estate that Henry said was struggling, which Henry takes as an insult. This is also in line with the Society scenes later on; Henry is clearly proud of his family heritage and doesn't want to hear it put down.
While Henry certainly never becomes a saint (see for example his two-faced treatment of Alexander), he does have one "redeeming" scene as early as in S1E6. He is the only classmate who tries to treat Wille normally and talk to him about something other than the video, and from this point on, his interactions with Wille get more genuine overall. He consistently tries to include Wille and ask him along in S2, and at least to me, the vibe is very different from that first dinner scene. It's not him sucking up to the prince anymore but pretty normal communication between casual friends. This is also supported by the fact that Wille in S3 seems to be friendlier with Henry than the average Forest Ridge guy (see how he taps Henry on the shoulder and says "see you later" in S3E2).
More importantly, there's also no more animosity from Henry towards Simon from S2 on. We actually see them hug to celebrate a floorball goal in S2E2, and even in S3 where Henry is portrayed as more elitist again, he doesn't start targeting Simon again. He actually seems sincere asking how the labour movement celebrates the First of May; the subsequent chuckle with Walter is amused, not mocking. Simon seems to have become a sort of class mascot or their "resident communist who always says such wacky things". In the S3E6 library scene, Henry blabbers about how Wille and Simon both need to come to the white party because it's all their last night together as bros, and he seems to genuinely mean it. So, you know. Some nuance there.
As for Stella... In that very same classroom debate in S1, her comment on welfare scammers with the dirty look at Simon and Sara is completely unprovoked. The same goes for her meaningful look at Fredrika when Sara tells Felice she might have to leave Hillerska; they are already portrayed as mean girls who only tolerate Sara as Felice's pet project. Yes, they are nicer to her in S2, but the moment Felice's opinion of her changes, they drop her and start badmouthing her without even wanting to hear her side. Never mind that Sara was kind to Stella and kept her secret about her crush on Fredrika.
I do want to say one thing in Stella's defence though, and that concerns her treatment of Rosh in S3. To me, her attraction and flirting on the camping trip seem very genuine; it's Fredrika who sours things when she very obviously gets jealous. The subsequent "wow I can't believe you've never been to the US, travelling is like mine and Stella's favourite thing" reeks of staking her claim and one-upping the competition, as does the way she keeps saying she couldn't possibly know Rosh hadn't been there when the camera pans away.
Stella's behaviour at the white party is appalling and confused me at first, but someone here on tumblr managed to spot a potential explanation: shortly before Simon, Sara, Rosh and Ayub arrive, there is in fact a split-second shot of Stedrika already making out. So while Stella definitely should've been nicer in rejecting Rosh, her bitchy bluntness can be read as her showing Fredrika that nobody is coming between them now. (The lack of focus on Stedrika really annoyed me in S3. Far too much of their potential storyline and Stella's character development/regression was left up to our interpretation.)
I don't have as much to say about Fredrika or Walter, because neither of them is fleshed out much. Walter is portrayed as a pretty obnoxious gossip from start to finish, but he does not actively bully anyone. If anything, we see Vincent make fun of him. There's very little else we find out about him, except that his family are likely business owners who avoid (or even evade) their taxes.
Fredrika is an interesting one because she acts as a sort of social gatekeeper for the girls just like early-canon August does for the boys. I believe it was @bluedalahorse who first pointed out how she is always quick to explain the traditions of Hillerska or the tenets of high society and hold others to them. Whether that makes her toxic is a matter of opinion; for me personally, it's less of an issue than her consistent mean-girliness (as outlined above).
So those are my interpretations of their in-canon characterisation, in good and bad. I'm sure others have their own.
Which brings me to my second point: when it comes to these background characters, the fandom's appreciation of them also stems from how they've been explored and expanded on in fanfic.
As the author of the first post-S1 Henry (and Walter, but especially Henry) fic, many people have told me over the years that the fic also affected their opinions on his canon counterpart. That was not my intention as such; I chose him because he was perfect for Outsider POV of Wilmon, because I was fascinated by his starkly different treatment of Simon and Wille in S1, and because I wanted to give him some depth and growth as a character in the context of the fic.
I can only speak for myself, but other authors will have had their own reasons for writing about these characters. Their fics will have similarly affected their readers and fuelled all kinds of hcs and personal attachments to these four that are not immediately obvious to the rest of the fandom.
In Stedrika's case, there was also a desire in the fandom to see more WLW in the show after S1 that made them a reasonably popular non-canon ship. Yes, Rosh was there too, but she didn't have any interactions with girls on screen; her queerness was still just a fan theory based on the actress, Beri. Madirosh did become a fanon thing early on, but there was little chance of seeing them become canon, so I don't think you can blame people for wanting to see some WLW among the Hillerska students. (Salice already had their shippers too, but there was also a quotient of fans who figured Sara might be aroace pre-S2.)
Anyway, I think it's more than fair for fans to develop their own hcs and attachments, even to controversial characters! Where it starts to give me mixed feelings is when fanon starts to bleed into canon, and I think that did happen in YR S2 and S3 to some extent.
Please correct me if I'm wrong, but I seem to remember that Lisa or possibly someone from the cast or crew said Stella's crush on Fredrika in S2 was inspired by the fans wanting to see them get together. I also think there is a very high chance that the sudden change in Henry's treatment of Simon between S1 and S2 was affected by the fans having adopted him as a favourite, and that the meadow scene between him and Walter in S3 was added as a nod to the Walty shippers.
And all that is nice, I guess, but it also has its pitfalls. If it isn't done carefully, it adds inconsistencies that may confuse and bother fans who don't enjoy these characters. This was very clear in S2, when a lot of fans actually felt that there was too much focus on Stedrika. In reality their screen time wasn't even that high, but it felt that way to many who would've rather given it to Wilmon, which again may have contributed to the lack of Stedrika in S3 that I mentioned above. It's also evident in the way Henry for example is still viewed by those who needed to see him apologise to Simon before they started to get along on screen - which I as a Henry writer agree was a very frustrating omission.
So in conclusion, anon is not wrong to be critical of these characters and confused about people's appreciation of them, and those of us who enjoy them are not wrong either. But I do think the writers of the show could have been a bit more consistent to help us keep track of what is and isn't part of their canon arcs and characterisation.
