há alguma explicação do porque foi removido do Dolphin o DX12?
Acredito que foi por causa da implementação dos Ubershaders 2.0 no emulador.
Ubershaders 2.0
Shader compilation stuttering is one of the most complained about bugs in Dolphin, so after Ubershaders development stopped, people didn't forget about it. The pull request, though long abandoned, still saw comments, got linked around the forums and even posted on the bug tracker in various forms.
Ubershaders was the first real hope to eliminate shader compilation stuttering, and it was still brought up on a monthly basis. If anything, the progress on it only inflamed the community's desire for a solution. After much pleading, begging, and much, much blackmail honest coercion, Stenzek reluctantly took over the mantle of Ubershaders.
Even before Stenzek began working on Ubershaders, the team made some decisions toward maintainability of the graphics backends. One of those decisions that was met with a mixed, if not negative, reaction was the removal of the D3D12 backend. Unlike D3D9, we didn't go through a deprecation process; we removed it once it was obvious no one was going to maintain it.
This was a fortuitous decision however, as the removal of that backend aided with the rebase and revival of Ubershaders when Stenzek was ready to give it his best shot. As he was the architect of Dolphin's Vulkan backend, he was already more than willing to go through the extra work to setup ubershaders to work on Vulkan.
When the pixel and vertex Ubershaders were finally hooked up together and ready for a run, testers immediately took them to some of the worst case scenario titles. Considering that none of the previous solutions really worked for a game like Metroid Prime 3, it was first on the docket.