Here’s what I would suggest: could the Blender Foundation, or someone maybe reach out to Apple and float the idea of Apple themselves developing the Metal 2-based version of Cycles, and making it available for download, for free or otherwise, say, on their Mac app store, as a plugin? After all, that’s what AMD did with their Pro Render: they distribute and maintain it themselves. And if Apple needs an incentive, let’s mention it to them that there are plenty of Blender users who love their Macs and don’t want to give them up, but without help, they may have no choice, because no one else can afford this effort on behalf of Apple, this time. Maybe Apple will take it up: after all, it’s their user base.