I used 4chan a lot and got to know a lot of people of a subcommunity that started there nearly a decade ago, and "4chan has become an alt-right hellhole and I don't go back because of that" became a common enough position among them over the last few years. I get the impression from a few of the progressive online communities I've dipped into that there's a number of ex-4channers that got sick of the culture shift at 4chan, but it's hard to quantify. A lot of people on Mastodon use it specifically because it has more protective rules than Twitter.

I don't think people like to talk too much about why they leave online hangouts, because it's like admitting defeat, because people still at the place won't sympathize with you because they either disagree with your criticisms or think you're slandering the place (or else they'd have already left too), and because outsiders who agree with your criticisms of the place may have other criticisms of the place and judge you for being associated with or expecting differently of the place.