Apparently, Prism Launcher chose to adhere to the idiotic principle of the hidden “trashbin”, .Trash-$(uid), invented by Ubuntu. Even though it’s based on QT. This can’t be disabled. It accumulated 139 GB of literal Trash, fully replaceable, over time. Just … why? There’s even an open issue about this, for over a year, referenced multiple times. I guess I have another point on my agenda.
Neat! I kept using du and df and work my way to the culprit. On PC’s with GNOME I use baobab.