AFAIK Metal is a separate API. That is, no more OpenGL, Mantle, DirectX, ShaderModel, etc. when using the Metal API.
The Apple Metal documentation is here:Metal Overview - Apple Developer https://developer.apple.com/library/prerelease/ios/documentation/Metal/Reference/MetalShadingLanguageGuide/Introduction/Introduction.html
It appears for 2015-2020+ for iOS and OS X Unity and Unreal will have to focus on output to Metal and NOT OpenGL or DirectX… so as to get maximum performance (and features) on those platforms.
Luckily for us, the “headache” is on the Unity and Unreal engine developers.
So for UE4 looks like the target APIs will be:
iOS + OSX - Metal.
BTW, this article says Apple may “destroy” OpenGL: Metal: Apple may destroy OpenGL and boost DirectX 12 - Developer Tech News …Which IMO is a bit premature since Android will carry the OpenGL torch for quite some time and over the next several years Android will attempt to transition from OpenGL to Vulkan.
OpenGL will eventually be deprecated by Apple, but that won’t be for quite some time so OpenGL will remain a target API for Mac UE4. Not least because it will provide backward compatibility with Mac users running Yosemite (10.10) and those using AMD 6xx0 series GPUs that don’t support Metal.