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?
Not quite. Other instances subscribed to remote instances are sent the information about new posts, comments etc and they store them locally on that instance. So, while there’s not be new content (since the main instance is the controller for all incoming content and distributes it back out, it would break the connection for new stuff.
There are manual steps an instance admin could take, to take it over. Probably it would need some agreement as to who takes it on.