A growing number of instances (mainly of Mastodon so far) are signing an ‘Anti-Meta Fedi Pact’, pledging to block any instance owned by Meta in the fediverse.
I don’t know how big this will get or how effective it will be, but if you run a fediverse instance, you should take a look at this https://fedipact.online/
I think the people in favor of defederation are looking at how Meta has handled similar situations in the past, and inferring from past behavior how they are likely to act in the future.