Those patches were never merged, and I doubt they will be, because there seems to be almost no interest in supporting high DPI displays on X without a compositor. If there was interest in those, at the very least somebody would have to adapt them and figure out how to use them with XWayland. In my opinion, any patch that doesn't fix this for Xwayland/Xwin/Xquartz is probably not going to fly. If you want this, you actually do need some level of protocol support for this, and I want to address a common misconception:
>X11 provides the necessary information via RandR to perform per-monitor scaling
This is not true. If it were true, it would be significantly easier to get DPI scaling to work on X11 and XWayland already. If you're referring to the physical size measurements, there are numerous problems with those, and they should never be used to do DPI scaling. If you're developing for Wayland, you should just ignore those measurements and use the scale provided by the server. If you mean that applications can draw themselves at arbitrary scales, they could always do that, without even bothering with RandR, but that will break down when you try to handle the compositing cases, e.g. stretching a window across monitors and expecting the display/input coordinates to scale correctly. So regardless of what you're trying to get the apps to do, some more work needs to be done there.
Even if those problems were fixed, DPI scaling would still not work without more changes in the server. The major reason why it can't even work is because of input coordinate scaling -- the server needs to be able to scale coordinates based on a factor provided by the client, and RandR does not provide this, and it doesn't make sense to add it to RandR either. If you want to standardize how this is done, the best way would just be to copy the method used by Wayland back into the X server. Part of it might be merging those patches by Keith that you linked, part of it might be somebody finishing this MR: https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests...