Earlier quoted context omitted.
As a positive example, Mastodon seems to work fine.
Other than playing whack a mole to block all the neo nazi and other hate filled instances, which are legion.
I could see why this might not scale as well to instances with thousands of users, or if a user is a frequent target of harassment (where opt-in federation might be the only solution), but for our lab, the existence of all the nazi or whatever instances has been resolved via a simple "Oh, these again. click, click, click There, the whole instance is banned." a few times a year. I have a suspicion that Dunbar's number or lower might be the optimal size for any federated social media instance, since knowing every user on my instance makes it a lot easier to tailor the little effort I have to put in.
[0] https://seirdy.one/posts/2023/05/02/fediverse-blocklists/