Even back in 2002 RandR supported obtaining screen size information in pixels and millimeters which can be used to calculate DPI... or actually, what people really want when mentioning "DPI support", an automatic way to scale the windows.
How the window scales is up to the toolkit to implement and is independent of window system and really, even independent of screen size/DPI since some users (e.g. due to sight problems) may want their windows to be scaled even on "low DPI" monitors (the whole DPI thing is just a default setting).
The main problem i see with X here isn't some technical inability, but a way for window managers to notify other clients that they should change their scaling (since the window manager is in a better position to know what scaling each window should use - especially when you want different scaling per monitor, which again can be independent of DPI since you may want a large scale for a wall mounted TV even if the DPI would be very low) - essentially something like a new _NET_WM_SCALE window property for EWMH clients and window managers.
Even back in 2002 RandR supported obtaining screen size information in pixels and millimeters which can be used to calculate DPI... or actually, what people really want when mentioning "DPI support", an automatic way to scale the windows.
How the window scales is up to the toolkit to implement and is independent of window system and really, even independent of screen size/DPI since some users (e.g. due to sight problems) may want their windows to be scaled even on "low DPI" monitors (the whole DPI thing is just a default setting).
The main problem i see with X here isn't some technical inability, but a way for window managers to notify other clients that they should change their scaling (since the window manager is in a better position to know what scaling each window should use - especially when you want different scaling per monitor, which again can be independent of DPI since you may want a large scale for a wall mounted TV even if the DPI would be very low) - essentially something like a new _NET_WM_SCALE window property for EWMH clients and window managers.