Is the Metal 2 API identical to the Metal 1 API? If it's not binary compatible forwards and backwards - it's a new API.
It is binary and source compatible, forwards and backwards. So.... not a new API.
(New features obviously require High Sierra just like any other API, but the existing features work on everywhere.)
There is no porting a Metal 1 app to Metal 2. Metal is just Metal.
Which Windows, Android and Linux systems support Metal?
You said every few months, so I was asking what is between Metal and Quickdraw 3D. Apparently nothing.