Okay, I did some more experiments (by fact driven by acquiring a new monitor 
) So, 1) I went back from embedded modesetting driver to amdgpu. Now it works just fine. 2) All the windows are 30- or 32-bit according to xwininfo/xdpyinfo. Firefox, vkdt and mpv are clearly showing 10 bit output. No other graphic-oriented apps, though. DT, RT, ART, and all the viewers I have installed strip the output to 8-bit! What a shame! Beside vkdt (which is hardly usable because of very low-level interface) no app can effectively use 10-bit.
To bottomline:
– X11 does work with 10 bpc, at least on AMD hardware.
– You are still unable to benefit from it, let alone games and video players.
– Facepalm…
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