Example, this makes the Linux content depot *only* downloadable on Steam Deck.
As I understand, the idea is so developers can give special changes just for Steam Deck, but the messaging for devs seems unclear so people keep tagging it thinking that makes it available.
You as a developer don't need to tick anything to make it available on Steam Deck.
If you have a Linux version - that will download automatically, the Steam Deck *IS* Linux.
If you don't have a Linux version, Valve's verification will check it on Proton and set it accordingly.