People seem to want to overcomplicate the fediverse. Think of instances like independent old-school internet forums each with their own mods and subforums. You pick the one you want to participate in and sign up. Yeah, you can pick more than one, and of course you have to sign up for each one independently of the other. They do not share accounts. Each forum is independent.

The big difference is that each forum can “ally” itself with another forum and your comments and posts are shared and saved among the allies, members of the other forums can interact with you because their comments are shared back to yours.

So in this case one forum (lemmee) shuts down, but all the shared info will still exist on the other forums, it doesn’t get deleted from the allied forums. It’s just not accessible to the OP on lemmee anymore because the original account and forum are gone.

I think saved bookmarks that are saved on an account will no longer work unless your app saves them externally.