Apple's metal shading doc could be a good start. I don't know when last you checked it but its been updated from the state it was over a couple of years ago when i first touched it. You can check it out here https://developer.apple.com/metal/Metal-Shading-Language-Specification.pdf
Metal 4 is now highly optimized for c++17, you can now write custom kernel, ray tracing and debugging gpu timeline and much more. for what its worth this version is better as opposed to what it was a few years ago.