One of the first games that seems to use that toggle is Demons Gaze.
The title is a bit misleading, because obviously you can play it if you copy all the files in there, then add the game as a non-Steam game and play it. It just cant be played via Steam officially, so when you download the game either via Steam Deck and even via Steam on the Proton Desktop, it will have 0 bytes from what I understood.