2 cents:
1) this folder is still in the destination directory, so it doesn't solve anything. if the goal is to hide unwanted files, their pieces must go to the "incomplete" directory.
2) I just encountered a dreaded bug when a WANTED file (selected for download) was moved to .unwanted. It stayed there even after it was downloaded completely. And then when I manually moved it to the correct location (quitting qbittorrent for the time), after restarting qbittorrent still didn't see it and started downloading it again, and even worse, it also deleted that file which was completely fine.
deleting data without asking should never be done under any circumstances, right?