Do they get adopted by other instances? Are they still accessible from other instances? Can you still post on them from another instance?
Edit: From my understanding every instance that deals with a community has a cached copy. Will that copy disappear after a certain time, because it can’t phone to home anymore?
Just started learning about the fediverse but I suspect everything goes down with the ship. Sort of creates a development opportunity for community driven backup utilities, though. Interesting problems are fun development challenges :)