The UHD restriction is not a technical one. Content producers and the rest of the media industry has strict requirements about streaming. UHD content uses proprietary DRM systems from Microsoft and Apple that are considered more secure than WideVine, which is why those browsers are permitted to watch 4k.
You can't watch UHD content on Edge for Linux, for example, because the necessary DRM isn't implemented.
So does this mean that the DRM is still at least partially implemented in software?
I also seem to remember that Netflix on Windows (don't know about mac) also requires hardware decoding support for HEVC1 (or whatever their codec is). I never got it to work with an intel GPU (8th gen udh630 — supposed to support it) but it worked with an AMD GPU.
Partially, for sure. However, even with hardware support you need some kind of licensed (signed?) binary interface with the DRM hardware to ensure that nobody is decoding a hundred Netflix streams at a time.
I don't know a lot about how everything works below the hood, but you can probably find some of the details on the Microsoft website [0] if you want.
You can't watch UHD content on Edge for Linux, for example, because the necessary DRM isn't implemented.