Fwiw the KWin fix has been merged and planned for 6.3, although I don't expect it to be backported to all older KWin versions.
Also, another use-case for having WM detection, is that the display mode emulation is truly only useful on X server with a window manager that is not Gamescope. Under Xwayland (which could be detected in a similar way), or Gamescope, the feature is not really useful and maybe even not desirable.