Ugggggh. As if graphics support on macOS weren't middling enough already. It's like they're trying to become as irrelevant as possible in that area. I could understand if they were deprecating it in favor of Vulkan. That would be in-line with Apple's history of aggressively pushing forward new standards. But by no means do they have the clout to coerce developers into their own bespoke graphics API that doesn't work…
One of the reasons this has been so feasible has been the fact that the engine (MOAI) uses GL ES to manage the framebuffer itself - giving the same look and feel on all platforms the host runs. This has been, honestly, revolutionary in terms of building a single app that runs everywhere.
This now becomes more complicated because the engine has to be modified for Apple platforms to use Metal, and represents another fork/splinter in the unity of the host itself.
I wonder if their decision to use a non-standard graphics API is due to them wanting to make this style of development a lot more difficult in the future - i.e. are Apple passively antagonizing the cross-platform framework builders in order to establish an outlier condition for their platforms? The cynic in me says yes, of course this is what they are doing ..