Is it still possible to interact with the communities (post, vote, etc.)? If yes, is it still possible to delete them if the ownership of the community (aka the top mod) belongs to an user from another instance?
I believe you can reply locally on your instance to dead communities, but that data would never be replicated to other instances. The fediverse isn’t really a mesh network, more like a hub and spoke on a per community level.
Lemmy works on vampire rules. When an instance shuts down, its 100 most prominent users turn to dust and blow away IRL.
I would assume if the instance shuts down this means at the next point of sync the instance will disappear from your radar, pulling all posts and accounts and communities from that instance.
Iirc the likes from your account on that instance’s posts might be either completely removed (because the post doesn’t exist anymore) or they are in limbo but invisible and inaccessible, although I have no clue how that would work.
I believe posts are hosted locally on the user’s instance, so I guess it’ll be a scream into the void, possibly not a way to view them on Lemmy (although may still be visible on Mastodon)
