Yesterday I saw a beehaw (I think) community thread which got locked by the beehaw mods but because it was federated ppl on other instances could still comment. I think !selfhosted would be still be up on your instance if lemmy.world went down.
I believe that any posts and comments that were pulled and stored on your server will remain but new posts or comments will no longer federate. I'm actually not sure if new posts would be possible at all but you could always test it by setting up 2 servers.