I'd wager Mastodon is ready for this. Abhorrent/illegal/spamming/offensive/offending instances can be defederated from to remove liability at a server-level, and users can self-moderate with "block instance" functionality from their client. Both sides are sufficiently equipped to filter their respective feeds.
It's the realm of pure fantasy to envision a world where the EU bans an original community project to force everyone into a Meta-designed fork. Mastodon users should be safe as long as they're free to run their own server and client software.
That's fine. Blocking their entire website from ever hitting your feed takes 2 clicks.
My point is that this really doesn't matter. If they use ActivityPub as-written, both clients and servers can stop Meta content from reaching their feed. If you just find Threads content annoying, you long-press the post and tap "Block Instance". If you're a site admin and have been given a legitimate reason to block Meta for API abuse (eg. poorly-moderated content, spam, advertisement) then you can exercise your defederation power and be within your right.
It's the realm of pure fantasy to envision a world where the EU bans an original community project to force everyone into a Meta-designed fork. Mastodon users should be safe as long as they're free to run their own server and client software.