To pick one of the list, OpenGL and Vulkan mesh shaders extensions were published in September 2018 [0], while D3D12 didn't support it officially until November 2019 [1] (on insidier preview of 20H1 build, which shipped in May 2020). Granted, there was NVAPI D3D12 extension kludge for it, but certainly Khronos APIs weren't treated worse. Talk introducing mesh shaders presented examples in GLSL, and they said they weren't covering NVAPI because it's ugly. [2]
Fair enough I was wrong on that one, with the small detail that on Khronos it was and still is, a NVIDIA extension, whereas all DirectX 12 Ultimate cards must support it.
[0] https://www.khronos.org/registry/OpenGL/extensions/NV/NV_mes... [1] https://devblogs.microsoft.com/directx/coming-to-directx-12-... [2] https://www.youtube.com/watch?v=Ge427_2VORo&t=1607s