I have added Steam Deck as it's own system. It's not obvious whether it is a separate game system or not, as it is basically a handheld PC running a Linux variant. My motivation for adding it as a platform is that even though Steam OS is based on Arch Linux, it is still a separate OS. A similar situation is the Xbox One that runs a modified version of Windows. Like a console, the Steam Deck has well-defined hardware specifications and controls. All the Steam Deck games already run on other platforms, but you could say the same for Google Stadia or Apple's tvOS, which I already decided to add as systems. I felt that playing the Steam games on the Steam Deck shouldn't be listed as playing them on Windows, which is just incorrect, and adding them as Linux games also seems wrong, if only because the games are running on a handheld system, which is very much not the traditional Linux gaming experience.
The first time I started Doom on the Deck, it ran at 30 FPS. I'm not sure if the cloud saves had restored settings from another session. I had to enter the advanced settings to reduce the graphical setting to Medium, and then the game ran in glorious 60 FPS. It still looks and plays great on the Deck.
Doom crashed with a Windows-looking error report window, that I can't close using the mouse, which is stuck in the top left corner.