For long spans of Mac OS X's history, Apple always shipped an incredibly old version of OpenGL. With Apple deprecating OpenGL, this isn't really anything new or interesting — that is, until the day Apple removes OpenGL, we will have the exact same situation we always had: a really old version of OpenGL.

In the mean time, people have already developed ways to run OpenGL code atop Metal. A lot of game developers have already started to move their code to either Metal or these OpenGL-compatible shims in order for compatibility with iOS, so a lot of the adoption has already begun.