I think they're expecting developers to use existing engines such as Unity and Unreal. Most game developers probably already use those engines are will not be effected.
On the other hand, they definitely made it harder for developers to create a new game engine.
Game engine not so much, Apple has always been an "also ran" when it came to games. Moreover, these days nobody is writing new major game engines - it is just way too expensive and difficult to justify vs. downloading Unity and starting building your game. Furthermore, games have always been something that is on the market for a year or two and then it is done for, with the developers moving on.
However, where it will have major effect is availability of any professional software. Such as CAD. Mac has always been a pain in the ... platform to support because of their weird "Unix but not quite" ways of doing things and now there will be no justification to support it anymore, especially in the OSS sphere. E.g. I fully expect things like KiCAD (and also the commercial Eagle which has Mac support) PCB CAD to disappear from Mac as soon as OpenGL is removed. Nobody has resources to rewrite such software to use Apple-specific Metal. Another such project is OpenSceneGraph, a large building block for 3D visualization and simulation applications.
On the other hand, they definitely made it harder for developers to create a new game engine.