So my #arch#linux just catastrophically self-destructed. I was using arch with the yubikey full-disk encryption package, when the machine hung and crashed during a system update. The machine crashed exactly after the old initramfs files were cleaned up, and before the new ones were written to disk. Since the yubkikey fde thing stores the seed ("challenge") for the luks key in the initramfs, all copies of the seed are gone now, and the data on that disk is unrecoverable.
Quite the failure mode if you ask me. I guess I will be scraping that yubikey fde thing from all of my machines now, and go back to plain passphrases. Deleting the old seed files before the new ones have been written and flushed to disk is a pretty bad design error.
Update to the update: The creators of the #arch#linux yubikey full disk encryption thing have responded to my bug report with what is essentially a shrug emoji and the line "I hope you had [a backup]".
I don't think that's an appropriate reponse from the maintainers of a critical piece of software like this. I think if you choose to release software like this, you have a responsibility to either make it good or to at the very least warn users that it's bad.