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.
My condolences on your lost data. That must have been a horrible realization.
I’ve also learned the hard way to keep all my important documents on Nextcloud which is completely synced to my desktop and laptop as well as backed up to another server with Borg. I also backup the ephemeral local files (screenshots, wallpapers, programming stuff) from my SSD, again with Borg to that same backup server and an external hard drive.