Yes, for the most part. All of their work on Proton (their fork of Wine) is open source, they're working with Codeweavers (the largest contributors to Wine) clo

Yes, for the most part. All of their work on Proton (their fork of Wine) is open source, they're working with Codeweavers (the largest contributors to Wine) closely, and contributing as much as they can back to upstream Wine. Part of what makes SteamPlay work so well is actually outside of Wine though - a lone developer started a project called DXVK a year or so ago, which is a separate translation layer for converting DX11 to Vulkan, and runs alongside Wine. Valve hired this developer and sponsored a few similar projects, and integrated them into SteamPlay. It also seems like Wine is slow and quite opinionated on merging in changes. For a while now, most people who used Wine for gaming would use forks and patches to improve performance. Some of it got merged into upstream Wine eventually, some things get rejected as it works differently to the usual Wine way of doing things. So Steam has used some of these patches and their own work for Proton, but some of it may not be suitable to be merged into mainline Wine.