I think the ideal solution would be devising some way for the content to live on cached in other servers and continue to sync comments and upvotes among them. It would be a shame for useful content to go away, and allowing for communities and instances to rise and fall and live and die seems like something decentralized content should expect to happen without leaving information holes.
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This would be much easier if the original owners of the instances helped bring it down. Their servers have private keys that verify ownership of the content for the fediverse to work in a trustworthy way.
Without these keys the content has to stay in a sort of read-only state.
My initial thought was to build an archive server that grabs all the federated instances' content via the public APIs. Then each instance would then have the option to purge after. However, this prevents folks from discovering it. Unless they are aware of the archive. I bought the domain lemmy.rehab for this. Perhaps, we could even create an instance that hosts only a backend for these dead communities that no one owns. Then moderation would be a nightmare.
The second option was post a sticky post that said this community was dead and no longer synced. But for how long is that useful? What if someone finds a post from years ago and comments and never visits the main feed to see it's dead?
Another option is to put in a request for the core devs to have archived communities. Read-only. However, these communities will remain in a constant out-of-sync status. Comments and posts on discuss.online will never show on lemmy.world or the other way around.
I'm not certain of any option.
I like 3. acknowledge it's dead, maybe provide a re-direct to a child community
This assumes they can find that post. They'd never know if someone searches and lands on a post and not the feed. Or if they only look at their main feed and never review the community.