It seems like it's ready to go in mailine. Linux doesn't really do "Big-bang" Everything is done releases of new features. Support is added incrementally. Here it says USB, PCIe, IOMMU, NVMe. Haven't been finished. But given those features are spread occross difference subsystems and have different maintainers it'll be easier to work on those once the platform is in Mainline. (You won't be able to expect all the PCIe developers to have the special M1 mac branch ready for testing)