IIRC, a big chunk of that was the trend that led to wayland causing "effective deprecation" of some more complex aspects of X11, like replacing multihead with windows into unified frame buffer (which now required single DPI for all screens while multihead allowed completely different settings)
Xorg does support having multiple monitors with different DPI, however applications need to use the RandR extension to obtain that per-monitor information as an older API that provided per-screen information (which doesn't make sense for Xinerama anyway) is hardcoded for backwards compatibility reasons (an attempt was made a couple of Xorg releases ago to remove the hardcoded value and present a more appropriate value but it broke a bunch of programs so it was reversed).
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The thing that made me switch to Wayland was having two monitors with different DPI, which was unsupported by X.
IIRC, a big chunk of that was the trend that led to wayland causing "effective deprecation" of some more complex aspects of X11, like replacing multihead with windows into unified frame buffer (which now required single DPI for all screens while multihead allowed completely different settings)
Xorg does support having multiple monitors with different DPI, however applications need to use the RandR extension to obtain that per-monitor information as an older API that provided per-screen information (which doesn't make sense for Xinerama anyway) is hardcoded for backwards compatibility reasons (an attempt was made a couple of Xorg releases ago to remove the hardcoded value and present a more appropriate value but it broke a bunch of programs so it was reversed).