Mine you at the time Valve was trying to get developers to make Linux ports of the games so targeting Debian made some sense in terms of platform stability, this didn't work out well and developers did no such thing. Valve then moved to making WINE work better through spending dev time adding patches and making the Proton later on top of it.Valve likely moved to an Arch base to get bleeding edge support for new hardware and for performance enhancements that come along with it as they were no longer shackled trying to get developers to make native Linux ports.

Thanks for review. Please post also to protondb.com to help users and devs there...

Proton is a new tool released by Valve Software that has been integrated with Steam Play to make playing Windows games on Linux as simple as hitting the Play button within Steam. Underneath the hood, Proton comprises other popular tools like Wine and DXVK among others that a gamer would otherwise have to install and maintain themselves. This greatly eases the burden for users to switch to Linux without having to learn the underlying systems or losing access to a large part of their library of games. Proton is still in its infancy so support is inconsistent, but regularly improving.