I don't see where Apple said they were abandoning OpenGL.

They just moved over to Metal for AppKit operations which naturally would always be faster than a common abstraction layer.

Apple more or less abandoned OpenGL years ago. The state of OpenGL on OS X is a trainwreck. It's outdated by about 5 years.

Just look at the supported version list: https://support.apple.com/en-us/HT202823

Note that there's a lot of hardware on that list that supports OpenGL 4.5 and indeed has drivers for that on other platforms but on OS X all top out at 4.1. That may not sound like a big deal, but OpenGL 4.3 brought full compatibility with GLES 3.0. That's a biggie if you're, say, an indie game dev that wants to do a cross-platform game that can span from mobile to desktop. And 4.5 brings the robustness support that allows for secure WebGL.

Could OpenGL be implemented as a cross-platform library on top of Metal/Mantle/Vulkan/DX12?

What makes you think Apple is going to keep their OS X GPU drivers up to date this time?

I'm sure OS X will retain its current support for OpenGL. But I think this means that Metal, not Vulkan, is going to be the equivalent to DX12 for OS X development.

I'm going to continue to get downvoted but oh well.

Apple was always going to support Metal on OSX in order to allow for iOS apps to transition back. It would be stupid not to.

That doesn't necessarily mean they are abandoning OpenGL. Just seems a little early to be getting the pitchforks out.