I imagine users go poof. Are their profiles stored in other federated instances? Is there a way to recover them or “import from backup” onto another instance?
If they don’t have an e-mail I imagine you can’t even notify them or authenticate them elsewhere so this “import from backup” even if technically feasible (idk if it is) would be impossible in practice due to authentication issues.
And communities, can you even notify all your subscribers to move to the “backup community” on another instance? I saw yesterday that a Mastodon server host said “I’m deleting this instance in 2 days” or something like that and I started wondering how shit would go on Lemmy.
Best would be if there is an app, that would do backups of your posts and content automatically, that if, for whatever reason, the instance goes down. You have first all of your data saved and second you could import it fast onto another instance.