Metal is supported, and used in production by a number of clients such as Flutter.
I don't think we have a particular link listing them all, but the GPU backends we support are:
* OpenGL/OpenGLES
* Vulkan
* Metal
* Direct3D 12
* Dawn (Google's WebGPU implementation)
I'd forgotten -- Vulkan is another option for Windows as well. However, if you do look at D3D12 please let us know. It's our most recent backend and we'd love for more people to put it through its paces. Last I checked it was faster than using ANGLE and D3D11.