I saw a post a long time ago saying it was vanishing due to rise of smartphones but now it has VANISHED. Authors of both art/fanfiction,books,etc generally can't handle beyond 'Cool' or 'Thanks' this has caused realistic authors to sometimes ask why they CAN'T get any lengthly reviews and I can tell you why if your one who seeks feedback:
THEY DON'T WANT TO GET THEIR HEADS BIT OFF!!! Simple as that. A02 is worse at this as it's all designed for authors to have full control so people are NOT going to give a detailed review knowing their comments can be deleted at a whim over something minor.
Hell I deleted my account as I wanted to do a short story but since it's mostly women and when they reveal where they are from the majority are Canadian and don't give a crap about how you feel if their character is a total dick. The character 'clashes' often seem forced to promote dividing like the author is trying super hard to force a debate instead of letting events play out naturally.
Writing has a low bar for entry given the large literacy rate and proliferation of computers and smartphones in the modern world. So you're going to have a large number of people who will attempt their own fanfiction, short stories, books, ect., and think that they can do it well given they've been writing for years in school. But creative writing is a different beast from regular writing and good creative writing is far above that.
The safe space/participation trophy/there are no wrong answers culture in school is another factor. When people jumping into something like creative writing only get told how good they are and how they can do anything if they put their mind to it hit the real world they flounder. They may have it in them to succeed but they were never told there was going to be a lot of hard work and missteps along the way. So those people default back to just wanting praise for their work and just consider everyone offering valid criticism as trolls or 'haters' or fallback to 'It's just not for you.'
Good writers, or people who want to become good writers, will need to group up and make their own writing groups where they offer valuable and effective criticism. I've heard it's tough to find a good group of writers for that and maybe some enterprising individual can come along and make a website for that one day. Then, hopefully, control of it can be retained from the leftists that'll want to take it over and turn it into another safe space.
Also, by AO2 do you mean Archive of Our Own?
Yup! Archive of our Own is worse I THINK then FF.net has for as many rude authors I have also gotten some that are interested.