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?
A reasonable alternative would be to implement OpenGL on top of Metal for compatibility but this is a lot of work.
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]