They do and in fact both X11 and Windows are capable of using that information to scale font pixel sizes appropriately. In both cases this mostly defaults to of

They do and in fact both X11 and Windows are capable of using that information to scale font pixel sizes appropriately. In both cases this mostly defaults to off and fixed DPI for UI font layout because the result of non-default display DPI leads to either the result looking wrong/ugly (blurry text, thickness of font strokes disproportionate to other UI elements etc) or even unusable (there is edge case in Win32 UI where one way to create non-resizable dialog-style top-level application window causes its contents to scale with DPI, while size of the window itself does not -> you get UI elements that are cliped outside of the window).