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?
From memory posts are partially archived by Google’s cache, so if they’re indexed correctly people should still be able to search for something and have it as a result? Unsure if that would work if the whole domain is actually gone though