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?
The accounts are gone-
But, communities, posts, comments are all replicated. With a single flip of a bit, you can take that cached copy, and turn it into a local community.
Users would need to re-subscribe/follow/etc, but, all of the data would be there.