XWayland isn't going anywhere any time soon and the main difference between XWayland and Xorg (the standalone X server) is the module that talks with the underlying hardware, which is only a small part of the otherwise shared codebase - and said module is basically using the same DRM API like pretty much every Wayland compositor and it isn't really that complicated to keep working (or even that big in size).
As for HDR i think the main reason you don't see it on X is because at this point there isn't much of an incentive for anyone who can both work with X11 and has the time to do so and has the necessary hardware. Personally i believe i could work on it, if i had an HDR monitor[0] but chances are even if i had an HDR monitor i'd only be using its HDR mode for games - and at that point it'd be easier to just switch to another virtual terminal and run the game via Gamescope (which already supports HDR).
I'd expect HDR to come to X11 much faster once mixed content between HDR and SDR starts becoming common on the Web as that'd require the X server to do the color space transformation for the (otherwise SDR) window regions that display HDR content (and probably need some update to use the atomic modeset API to seamlessly switch between HDR and SDR, something Xorg doesn't do now but again, without the need to mix HDR/SDR there isn't that much of an incentive).
[0] ok, i have one but it is one of those "barely HDR" HDR400 monitors, i only tried its HDR mode under Windows when i bought it, saw it looking quite meh and never tried it again since i bought it a few years ago
As for HDR i think the main reason you don't see it on X is because at this point there isn't much of an incentive for anyone who can both work with X11 and has the time to do so and has the necessary hardware. Personally i believe i could work on it, if i had an HDR monitor[0] but chances are even if i had an HDR monitor i'd only be using its HDR mode for games - and at that point it'd be easier to just switch to another virtual terminal and run the game via Gamescope (which already supports HDR).
I'd expect HDR to come to X11 much faster once mixed content between HDR and SDR starts becoming common on the Web as that'd require the X server to do the color space transformation for the (otherwise SDR) window regions that display HDR content (and probably need some update to use the atomic modeset API to seamlessly switch between HDR and SDR, something Xorg doesn't do now but again, without the need to mix HDR/SDR there isn't that much of an incentive).
[0] ok, i have one but it is one of those "barely HDR" HDR400 monitors, i only tried its HDR mode under Windows when i bought it, saw it looking quite meh and never tried it again since i bought it a few years ago