I’ve seen some posts mentioning LiveJournal as an example of what happens when a site starts getting really aggressive about policing content, but this particular situation is making me think of Quizilla.
Quizilla was the weird little site that introduced me to fanfiction. It was supposed to be for personality quizzes, but in practice it had a mix of fanfic (sometimes in a semi-CYOA form), original fiction (sometimes in that same semi-CYOA form), and quizzes. Some of it was smut. Writers would generally warn you that a “lemon” chapter was coming up, and there was an “are you sure you want to see this mature content?” screen you had to click through before you could read anything too iffy. It worked fine, at least as far as my tiny self could tell.
Then they got bought out by Nickelodeon, who went “oh no, this is a teen site now, and it must be Made Appropriate for Children!”
So off went a good deal of the lemons (without any warning, as far as I can remember). Some people still tried to sneak smut past the moderators, but what they got away with was far tamer than most of the things you’d see on Ao3.
This did not stop Nickelodeon from trying to keep “fixing” the site, with disastrous results.
The tag system was pretty much permanently screwed up at some point– I can remember scrolling through pages and pages of content marked “angel” or “demon” on the old Quizilla, but after a certain point of the changeover, I only ever got one short page of results for each tag, even when I knew there was more to be found within that tag. A lot of older stories, most of which weren’t lemony at all, simply ended up butchered after Nickelodeon decided that stories could no longer be in quiz format– basically, they imposed new word count limits on the quizzes, and they just deleted any parts of the story that overran the word count. They got rid of the forums because they didn’t want to bother moderating them, and I also seem to recall them banning custom themes for profiles, quizzes, and stories at some stage.
It was a slow process, but a lot of works were lost, both good and bad, both smutty and PG-rated. And eventually, once they’d screwed up the site and driven off a lot of people, they shut down the site.
tl;dr: Dammit, I already lost one guilty-pleasure site full of fandom content because of misplaced attempts to get rid of porn, and I don’t want to see this site start making the same mistakes.