I believe that Hawkins started getting flayed after season 2, especially after the Snowball '84. Something has changed in season 3.
My main reason is that Will was slowly pushed to the edges of the supernatural narrative in season 3 and totally in season 4. His sexuality arc was more apparent in these two seasons and while I absolutely have no problem with that, it's obvious that his connection to the UD was reduced to goosebumps and the hivemind. His trauma was also not explored deeply. His connection to Vecna was only shown in season 5. Idk if this was supposed to be a plot twist though.
Anyway, we know that Will overcame the Mind Flayer's possession in season 2 when his loved ones reminded him of their cherished memories with him. His birthday, Castle Byers, Mike and Will's first meeting and becoming friends... It is essential that Will is reminded of the fact that he is loved and remembered. And that is what made the MF to lose possession of Will ultimately.
And in exactly the next season, everybody is forgetting Will and moving on with their lives? Even Mike suddenly wanted to be grown up and not spend time with his friends anymore.
I'm not trying to sideline Mike's character arc, or any other characters' journey, in any way! So hear me out.
Will was forgotten by his friends; Mike was being pretty rude to him and he was going through his trauma alone while still grasping at DnD to live a childhood he was forcefully robbed of.
Will destroys Castle Byers, which has been his sactuary for years, in an attempt to 'grow up' and not be 'stupid' anymore. And remember it's the building of Castle Byers that Jonathan reminded him about in season 2. But then... Castle Byers is never mentioned again. Never. Not even Will thinks about it. How come?
In season 4, Mike arrives at the airport on March 22nd aka Will's birthday date.
Joyce, Jonathan, Mike, Dustin, Lucas, nobody remembered it. This is yet another memory Will is reminded of in season 2. (I'm still skeptical about the exact date of Will's birthday I must admit) Everybody is forgetting him. It's highly unlikely to me that Mike just forgot his birthday. He noticed every single microexpression of Will's at the Rinkomania so he was not ignoring his boy.
Then comes the "it was the best thing I've ever done" scene... While we know that Mike was not being completely truthful in the 'confession scene', does Will know it?
Again I'm not saying that the other characters' actions ultimately lead to Will's own arc. Mike is going through his own problems, just like Jonathan and Joyce.
But the emphasis is there.
We are shown Will destroying Castle Byers as an act of self-deprecation and anger at himself. We are shown the date at Rinkomania on the recording, reminding us that it was Will's birthday. We are shown Mike and Will side by side on the same frame as Mike 'confessed' to El. We are reminded of Will and his memories, all taken from him. Leaving him vulnerable and forgotten. Like he is being erased from the narrative.
Will is constantly made to feel like he is insignificant, that he is not accepted. And isn't that exactly what the Mind Flayer wants? To take over his mind and body? To make him another Henry Creel.
