The story of Apple deprecating OpenGL on macOS is the same story as Apple making a mixed framework to allow iOS apps to run on macOS.
iOS runs all its graphics on Metal, and now macOS has converted all their window servers and graphics to Metal. That means mixing display elements and windows from iOS on macOS is possible in a sensible way. Apparently having any OpenGL running on an macOS machine and trying to also show an iOS app with Metal is so difficult they’d prefer not to try it.
Games that use the big engines like Unity, Unreal, etc will be fine. Game companies that use their own technology in that regard will have to invest making it work on Metal or, more likely, not bother with supporting macOS.
However, to get a game running on iOS means using Metal anyway, and next year it will be possible to write one game that works on both an iPad and a MacBook. I think Apple is trying to steer developers into making games and apps that can take advantage of both the high power and screen estate of an iPad Pro, and that will also happen to be a powerful Mac app too.
It’s not the case of “drop support for macOS” but “we are making supporting macOS as easy as writing an iOS app”.