Proton 9.0 Beta 11 was released today and is now available for opt-in via the Steam client. While "Beta 11", this is their first public beta of the new version. As implied by the version bump from Proton 8.0, they have re-based from a Wine 8.0 base to the recently released Wine 9.0 .
In addition to pulling in the Wine 9.0 codebase, the Proton 9.0 beta also updates its bundled versions of VKD3D-Proton, DXVK, and DXVK-NVAPI against more recent snapshots. Games now known to be playable with this beta Proton 9.0 code are Dinogen Online and the Photography Simulator demo. There is also a number of games previously only playable with Proton Experimental that should work with Proton 9.0: The Finals, George McGeehan Gamer Hero, Road to Vostok demo, True Reporter. Mystery of Mistwood, Witch On The Holy Night, and Lord of the Rings: Gollum.
Proton 9.0 beta also brings many game fixes, improved video playback for numerous games, improved support for input devices having 8+ axes, updated the file distribution method to conserve disk space, and many other changes and enhancements for bettering the Linux -- and SteamOS / Steam Deck -- gaming experience.
Those rolling their own Proton builds can find the 9.0 beta via GitHub along with the official change-log.