I guess it somehow makes sense for Valve to keep the per-game tweaks private. With all of that money they're printing though, I sure wish they could feel that they could afford to be less defensive.
None of it is private. The per-game settings are all listed here, the options are all implemented in Proton, which is completely open source: https://steamdb.info/app/891390/info/
Steam Play is part of the Steam client, so is proprietary. ProtonDB isn't open source but does data dumps to github regularly [3].
[0] https://github.com/ValveSoftware/Proton/
[1] https://github.com/doitsujin/dxvk
[2] https://github.com/Joshua-Ashton/d9vk
[3] https://github.com/bdefore/protondb-data