What happens if an Fediverse instance goes down?

Based on what we saw when beehaw de-federated from lemmy.world, communities still exist on all remote servers that previously saw it, including any posts that were already there.

The only unknown is if those communities will exist indefinitely. I don't think there is currently any mechanism for cleaning up instance data but there might be in the future. It would be a good idea to have that as an explicit option for admins.