SteamOS 3.5 has just been released to the Preview channel, and includes new features that are still being tested. You can opt into this in Settings > System > System Update Channel. Display The default color rendering for Steam Deck has been adjusted to emulate the sRGB color gamut, resulting in a slightly warmer and more vibrant color appearance. Added Settings -> Adjust Display Colors, to tune the display's Color Vibrancy and Color Temperature.
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.
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.