MonoGame does not support Metal at the moment, but FNA has a Metal graphics backend that is almost ready to ship. So if the hammer comes down on GL support sooner rather than later, you can just use FNA for iOS/macOS/tvOS. In theory it should be a very simple transition, since the vast majority of the API is shared between the frameworks.
That said, it'd be great for MonoGame to adopt Metal too, especially since Apple is likely to force the issue sooner rather than later. If anyone reading this wants Metal support for MonoGame and has an actual budget, please reach out! This is a project I'd be more than happy to work on.
(Side note for those in this thread advocating for a Vulkan backend: IMHO, Metal is actually really nice to work with and leagues better than Vulkan in terms of API design. The MetalDevice I linked above has ~6k LoC and can run complex games like Celeste and FEZ. I tried writing a VulkanDevice and gave up after writing 7k LoC that still couldn't even clear the screen. It's not an exaggeration to say there's an order of magnitude difference in complexity between the APIs. That said, if Tom & co. can pull off a Vulkan backend, more power to 'em!)