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?
As of Lemmy 0.18.1, cached copies on other instances do not disappear if the original instance has died.
In theory, it might even be possible to actually clone a cached copy into a new local community. This would require some database hacking, so not recommended unless you’re familiar with Lemmy code and SQL.
It’s actually extremely easy to flip a remote community, into being a “local” one.