At least for Lemmy -- I have no idea about kbin or other ActivityPub software -- there isn't a user-accessible way to back up one's account on an instance, nor to preserve any communities that you're a mod for. So yeah, if the instance goes down unexpectedly like due to data loss or an FBI raid, the communities and users that were on that instance will disappear.
It's true that other servers will have a cache of some of the existing community posts and the users on the departed server. But it's exactly that: a cache, which will eventually be evicted.
A similar situation occurs when a Lemmy instance changes domain name: all prior posts to the community (and the community itself) were homed to the old domain. So a new domain cannot have the same identity as the old; it will simply be a separate entity, even if all posts were somehow preserved and reposted on the new instance.
Is this Lemmy-specific? No, Mastodon and I think all other ActivityPub software, plus BlueSky have this property, because they anchor identities to DNS names. From that, the posts to a community are anchored to the instance, and the instance is anchored to DNS.
So if the domain is lost, then it's game over. But if the domain is still there but the disk got wiped, then it would be a matter of recovery from a backup. You do have a 3-2-1 backup strategy, right?
I will note that Mastodon has a user-initiated export feature, which functions as a backup, something that Lemmy doesn't have. A Mastodon user can export their data and then move their identity to a new instance. Lemmy can't do that today, but it should be possible. Though in both cases, only the saved account is preserved. To restore a Lemmy community would require a disk-level backup image.
(this is all conjecture based on my limited knowledge of Lemmy. A better answer would come from an instance admin or one of the Lemmy devs)
Was this question also posted a few weeks ago?
In any case, what exactly are the requirements here? You mentioned encrypted journaling app, but also gave an example of burning a handwritten sheet. Do you need to recover the text after it is written, or can it simply be discarded into the void once it's been fully written out?
If encryption is to protect the document while it's still a draft, then obviously that won't work for handwritten pages.