I think most of the work Valve does is upstream from Arch though but it's trickling down very fast (with arch, sometimes hours, if it's an AUR-Git package, seconds).
https://www.youtube.com/watch?v=h7YbqrJ0_nM
PS: The core of Valve's technical contribution is around the SteamDeck. You already know it runs Linux, Proton, yadayadayada. An obvious fact that only clicked for me after being mentioned in the presentation: if you aren't in the Steam store but want your app on the SteamDeck the primary option is a Linux version (distributed through flatpack). That (commercial) incentive alone is a huge benefit. Suddenly you can make a business case why your product maybe should support Linux.
For anyone reading this: virtually every piece of code is upstreamed to the projects themselves (Mesa etc.), there is no reason to jump to Arch for gaming.