Today was Apple’s annual developer conference WWDC with the usual swath of new products and betas announced. Among the announcements was one of direct importance to game developers everywhere.
From the document What’s New in iOS 12:
Deprecation of OpenGL ES
Apps built using OpenGL ES will continue to run in iOS 12, but Open GL ES is deprecated in iOS 12. Games and graphics-intensive apps that previously used OpenGL ES should now adopt Metal.
Metal is designed from the ground up to provide the best access to the modern GPUs on iOS, macOS, and tvOS devices. Metal avoids the overhead inherent in legacy technologies and exposes the latest graphics processing functionality. Unified support for graphics and compute in Metal lets your apps efficiently utilize the latest rendering techniques. For information about developing apps and games using Metal, see the developer documentation for Metal, Metal Performance Shaders, and MetalKit.
They will also be deprecating OpenGL support in the next Mac OS version “Mojave”. Deprecating doesn’t mean that OpenGL will stop working on those platforms, but it may in future OS revisions. What it does mean however is that OpenGL support will no longer be updated, no new features, no new drivers, etc. So basically if you want high performance 3D graphics on iOS and MacOS devices, you now have to use their native API Metal.
If you want to develop cross platform for Apple platforms going forward you either need to use a game engine that does the work for you, or write a Metal renderer for iOS and MacOS targets. There might be some hope however in the form of MoltenVK which enables Vulkan code to run on Metal.