This. This is what I am looking for. I remember using iz3d years ago, with their anaglyph renderer. Playing Half Life 2, Halo and others in stereoscopic 3D was a great experience. Dead Space was particularly nice, as the game uses no HUD.
I guess most of it could run as a Vulkan layer?
You can already do a lot of it with ReShade shaders like the aforementioned SuperDepth3D, but it requires a game profile, tons of fiddling and non-incompatibility from the game. Having that shipped as dev-tested profiles with games would be the exact sort of thing that makes proton nice to use on the steam deck