There was a HDR hackfest earlier this year. A couple of reports from after the event if you're interested https://emersion.fr/blog/2023/hdr-hackfest-wrap-up/ + https://blogs.gnome.org/shell-dev/2023/05/04/vivid-colors-in-brno/. It also got a brief mention in the System76 blog https://blog.system76.com/post/may-flowers-spring-cosmic-showers.
So it's being worked on, and it seems all involved are trying to get it right - it sounds like gamescope on SteamOS doesn't need to worry about solving all the problems that general purpose desktop compositors will have to.
You might have just had bad timing - the release notes for Proton 8.0-4 mention
It's currently a test release so should be out shortly, but if you want to use it immediately there's some info on enabling it early at https://www.gamingonlinux.com/2023/09/proton-80-4-is-now-in-testing-with-a-release-candidate/