While HDR support is still not really there yet on Linux, there's a lot of movement on it now (and plenty of it thanks to Valve and work on Gamescope) but it seems KDE KWin may gain early initial support for it.
Developer Xaver Hugl put in a Merge Request titled "Add support for playing HDR games" that notes "This uses a protocol that's cut down from the upstream proposal, so that we can merge it quickly and use it for more extensive testing, and for playing games, until the upstream protocol is merged".
Seems like a good idea to get an initial implementation in, so people can actually do some proper testing and gradually build up the support for it on Wayland. Since it's an open Merge Request, it's not yet officially part of KWin and likely needs more eyes on it to check it over but it does include instructions on how to test it for those of you who really want to get in early copied below:
To test this, install https://github.com/Zamundaaa/VK_hdr_layer/tree/work/frog-protocol and a git master build of gamescope. Then enable HDR on your monitor if you haven't already, for example like this
kscreen-doctor output.1.wcg.enable output.1.hdr.enable output.1.sdr-brightness.500
and then run gamescope with
ENABLE_HDR_WSI=1 gamescope --hdr-enabled --hdr-debug-force-output --steam -- env ENABLE_GAMESCOPE_WSI=1 DXVK_HDR=1 DISABLE_HDR_WSI=1 steam -bigpicture
Note that a bunch of games, like Doom Eternal, require an EDID to parse for HDR to work. That's not implemented yet outside of gamescope's embedded mode, so HDR will not work in these games yet until that's done. Cyberpunk 2077 for example does not need that though, so it can be played without issues: