I'll upvote it with the hopes of seeing an answer from the devs. This could be a very important/useful feature.

Not in any real usable way other than an archive of what you can see in a browser. As of right now you’d need your own instance for a real backup.

Right now, no.

The best you could do is try to archive the updates on an instance you control, but that is going to require you running an instance, writing custom code, and possibly breaking any GDPR protections you might have by not cross-honoring deletions.

There is a reason why a lot of Reddit subs who want to make their own Lemmy community create their own instances.

Setup your own instance (don't even need to allow other members), create the community there. Then just create some alt accounts on other instances that you think would be interested in your community. Subscribe to your community from those instances to at least get your stuff to appear. Than hope for the best.

Can't answer it but isn't the whole thing backed up (synced) to other instances? If I'm browsing lemmy.world community on my lemm.ee account, I'm actually only fetching data from lemm.ee. Only downside is that if there is an instance started after the community got lost, it will not have that community synced.

While it's interesting, it's still more of a support question. Please check the sidebar for communities where you'd get more answers.

If an instance shuts down, there's still cached copies of the community out there in the fediverse.

My suggestion would be to back it up on the Internet archive manually once in a while.

Really only if the owner of that instance did a backup of the database following this here https://join-lemmy.org/docs/administration/backup_and_restore.html