Technically Xorg can handle fractional scaling across mixed dpi and refresh rate monitors, but it requires support from toolkits, window managers and applications which means the developers of all of those (or at least toolkits and window managers) need to cooperate. At minimum, you need toolkit support for the most basic. AFAIK Qt6 does have this support and should be able to handle fractional scaling across mixed DPI monitors but not in an ideal way since for better experience you'd need the app/toolkit cooperating with the window manager instead of the toolkit trying to do everything on its own without knowledge of the rest of the desktop. I wrote about it in detail here[0] but the gist is that it is largely an issue of getting the developers of various projects to cooperate than some feature Xorg itself lacks (it doesn't). Wayland had it easier here because it started from scratch and developers had to worry about those things for supporting it. On a more positive note, because of Wayland (and Win32, if a toolkit support scaling there) the the hard part of the work on the toolkit side should already be there.

For VRR the issue is how current desktop compositors render their output, though it should be technically possible to make a Xorg desktop compositor to use separate outputs for each monitor (may need to use Vulkan with custom barriers for vsync though, this is something i've only ). The alternative is to not use a desktop compositor at all, which is what i'm doing (since i also dislike the desktop lag introduced by desktop compositors). I have a 165Hz VRR monitor that i used it for a bit (even connected a separate 60Hz monitor for a bit) and worked fine, though eventually i disabled the VRR functionality since at 165Hz tearing is almost imperceptible (and it never bothered me even on 60Hz monitors anyway) while my monitor is one of those that have some annoying flickering with VRR enabled. In any case, the issue is with the setup and desktop compositor used, not with Xorg itself.

Of course from a user's perspective all these most likely do not make much of a difference.

For HDR there is no support for it Xorg though. Personally, the main use for HDR would be either some movie or playing a game, i.e. fullscreen apps, and switching to another virtual terminal running a Wayland compositor (or just Gamescope) just for those is perfectly fine - having to press ctrl+alt+f1/f2 instead of alt+tab is not a deal big enough to change the entire desktop setup i've been using for many years :-P.

[0] https://news.ycombinator.com/item?id=45858043