Long story short: by trying a bit too hard to clean up the server after the upgrade, I broke the system badly enough that it wouldn't boot again despite my best efforts.
(Just to be clear, the upgrade itself went just fine and you shouldn't be afraid of upgrading your own Debian based on my little mishap.)
In the end, I reinstalled Debian 11 from scratch and re-imported all user files. This had the added benefit that I did end up cleaning up a decade of cruft dating back to much earlier versions of that server.
There should be no noticeable difference to our users, except, again, a new SSH server key, whose fingerprint is now:
All my apologies for those two days of downtime — although averaged over the last decade, we're still around 99% availability. Nevertheless, if you have stuff on hisoka and it looks broken, or if something needs to be reinstalled, do yell at me!
I promise once all is working fine again, I'll try very hard to not touch anything for the next ten years.
We are a private domain hosted on a single server. Basically, if you can read this, tigris is online. Some of our users have publicly accessible sites, such as the furmap or the owner's blog.
Other than that, there's nothing to see here.
The tiger (Panthera tigris) is the largest living cat species and a member of the genus Panthera. It is most recognisable for its dark vertical stripes on orange fur with a white underside. It is territorial and generally a solitary but social predator, requiring large contiguous areas of habitat to support its requirements for prey and rearing of its offspring. (Wikipedia)
― Bill Watterson