/me wonders if OP has been paying attention to how "consensus building" actually ends up working in the Wayland world
> With Wayland since everything was done from scratch, there were less people that needed to be convinced to cooperate - and in practice since Wayland originated from RedHat and the GNOME ecosystem, convincing the appropriate GNOME and Gtk developers to cooperate was probably a coffee break away...
/me realizes that the answer is "No. Not really."
To be less droll:
1) Through xrandr, even windowmaker provides the data required for an application to know the properties of the monitor that > 50% of each of its windows are on. Given how much nicer xrandr was than xinerama, WMs that cared about multihead moved over to it fairly quickly.
2) I'm certain that not every WM provides the information required for screen-DPI- and screen-scaling-aware programs to scale as desired. But, the "Wayland is a lightweight protocol that makes few policy decisions" motto turns out to mean that for most decisions that users care about, each Wayland WM (or whatever the Wayland terminology for the Wayland equivalent is) needs to re-make and reimplement those decisions. Feature fragmentation has been bad. So, no, if you're not going to hold Wayland to the "Every WM must implement all the features" standard, then you're not going to demand that of Xorg WMs.
3) You happened to mention the two things that's needed for Xorg to support both HiDPI and non-integer scaling... GUI drawing library support and a common protocol for setting and retrieving user-driven adjustments to the "natural" rendering scale given the display DPI. XRandR [0] has either always, or has effectively always provided the information required for GUI toolkits to scale their widgets according to a screen's DPI. And the XSETTINGS protocol [1] is used to store the user-commanded scaling adjustment. Glancing at the release date for those two things, they either substantially predate or came out very, very shortly after Wayland's initial release.
Weird. It's almost as if we were waiting on the GUI toolkits to use what Xorg had been providing them for ages.
Anyway. Check footnote 1 in the comment you replied to for the on-the-ground details on GUI toolkit render scaling on Xorg from an end-user's perspective.
[0] adopted no later than 2007
[1] first proposed in 2001 and adopted no later than 2009 (though, if I cared to spend more than a few minutes on the search, I expect I'd find that it was adopted much earlier)