There was a HDR hackfest earlier this year. A couple of reports from after the event if you're interested https://emersion.fr/... + https://blogs.gnome.org/.... It also got a brief mention in the System76 blog https://blog.system76.com/....
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.