From what I can tell, an instance is either ‘linked’ (federated) or ‘blocked’ (defederated) on Lemmy. Mastodon has some more granularity. If an instance came back as a zombie, it wouldn’t be any more powerful privilege wise than a new instance that is malicious. It would get defederated same as always.
What could be a problem is on the individual user level. Say that a lot of users sort their feed by subscribed. They are not affected by random instances coming and going. However, they will be affected if a bunch of their (dead) subscribed communities suddenly become malicious.
Anyway, I hope this at least serves as a reminder to not let our domains expire ;)
It’s an important point for sure.
Your sensitive data and logins are tied to email addresses, which are tied to domains. Lose your domain, someone can access everything.