Metal is only supported on iOS, macOS, tvOS.
You might want to put it easier - Metal is only supported on Apple platforms.
Metal is pretty much Apple's version of Vulkan API (started by Khronos Group).
Presumably, it has the similar functionality, with some Apple's needs on top (such as using Objective-C on the top).
Apple didn't adopt Vulkan API, because it started its own development either earlier (or in parallel with Vulkan API).
Metal API was initially released in 2014, while Vulkan API followed only 2 years after that in 2016.
That makes Metal quite easy to use in FPC world, since it's supports ObjC out of the box.
The only issue is to have Metal headers handy.