AFAIK we’ve not defederated with anyone
There are, like, CSAM and Nazi instances.
I can only speak for myself, but I never see them in my All. Likely because I just block and move on when something I deem genuinely misanthropic pops up.
EDIT: I also have NSFW turned off, not sure if that also would be a factor.
apparently they really haven’t defederated from any instance, and also holy shit there are a lot of Fediverse instances