[account42]

They use gamescope under SteamOS, not Linux in general where you already have another compositor. Running all games under a nested compositor (gamescope) would have some perf impact (even if a small one) as well as an unexpected cursor acceleration profile.

[Cloudef]

True. I personally have setup my distro to launch directly into gamescope optionally for SteamOS like experience. On desktop client they still either hook or use vulkan layer (latter often in proton, thanks to dxvk). However I would not be surprised if they started using nested compositor even on desktop.