Isn't the concept of 3D APIs limited to stuff like Mesa, with the kernel space AMDGPU driver providing only the primitives on which the API can then be built? I could be wrong, but that's how I understood it.
I things like Fglrx, the OpenGL stack was part of the driver proper IIRC.
But granted, I haven't heard of or seen FGLRX in literally more than a decade, and describing AMDGPU as 2D only is misleading or pedantic.
It's fully accelerated, incl. video decoding & HDCP. Full stack sans the card firmware is open source.
The video decoder block is completely independent from HDCP block on the silicon to be able to provide open access to these parts too.
I don't know why they can't/don't open source video encoder parts. Probably some 3rd party royalties, but I'm not sure.
If you want another 3D API, you can directly build it on top of the driver. Nothing prevents that. Plus, ROCm packages are landing to "free/main" part of Debian for quite some time.