I finally managed to decrypt an old backup of an iPhoto Library.
The result was a file without an extension (maybe I removed it back then?) and the following header:
./._iPhoto Library
Now I have no idea how to open that library. Photos.app won't open it (start with option + shift, choose library), the iPhoto Library Updater won't either. It was definitely a library that you could open as folder (right click, show package content), I think from iPhoto 11. Is there a way to tell macOS that this is a package file? Or even a folder instead of a file?