Comment 26 for bug 313514

(In reply to comment #24)
> Yet another idea, how to make it possible for X11: X Keycode Extension.
>
> It would need patched of libX11 and server code on both sides. If X Keycode
> Extension will be present on both server and client, then wide keycodes will be
> transfered via extension. If not, low keycodes will be tranferred via standard
> X keyboard events, high keycodes will use "random" translation or they will be
> lost.

Given that this requires updates to XKB and Xi as well as the API and ABI of a lot of libraries, it's a lot easier for it to just be Xi 2/3 and XKB 2.