I'm curious about this – does it really matter? How important is it that the Operating System has OpenGL? Can't individual apps just static link to (and ship) their own versions of OpenGL?
OpenGL is just an API, the underlying features are provided by the graphic driver. You can't ship with your own OpenGL, that would mean shipping with your own amd/nvidia/intel driver (and the associated kernel module, etc).
A reasonable alternative would be to implement OpenGL on top of Metal for compatibility but this is a lot of work.
What is gonna happen to games and apps which were using OpenGL since today then? They will stop working in the new macOS version?
edit, ups just read it: Apps built using OpenGL and OpenCL will continue to run in macOS 10.14, but these legacy technologies are deprecated in macOS 10.14. Games and graphics-intensive apps that use OpenGL should now adopt Metal. Similarly, apps that use OpenCL for computational tasks should now adopt Metal and Metal Performance Shaders. [0]
