After a single APT command gone wrong made my Debian installation unusable, I decided to reinstall Linux. I tried to back up everything to my external hard drive, but it kept unmounting, so I elected to use Filen (a FOSS cloud storage provider) instead.

It was only after installing openSUSE Tumbleweed with KDE Plasma that I realised I hadn't actually synchronised the folder I had moved my backup to; meaning I have lost everything but a single Minecraft world (which I had backed up to a Compact Flash card in February).

Tl;dr: Double check your backups, and use physical storage whenever possible.