Shouldn't every GPU that fully supports DirectX 12 support all of its features?
I want a pony as well. GPUs aren't like CPUs which are all the same.
GPUs are extremely different, and some of them have superior architectures than others, some are better at doing certain tasks, other are better at other tasks.
Specially when you want existing GPU hardware to be able to run DX12 right now.
If you don't like that, then you can get out of graphics development in games because this heterogeneity has been driving innovation for the last 2 decades.
To the OP:
There is a chart with tiers based by GPU.
Don't be fooled by them though. A tier 3 GPU may be tier 3 because it doesn't support X & Y features, but turns out if it weren't by them, it would be considered Tier 1 (e.g. it may have features or precision that not even tier 2 GPUs have).
Tiers only guarantee a minimum, not a maximum. You should watch out for the capabilities you can query via the D3D12 API.