Apple widening NVMe flash storage support in High Sierra possibly good news for Mac Pro, iMac Pro
Discovery that Apple has intentionally removed restrictions on NVMe in the High Sierra beta suggests that future Macs won't be limited in which mass-storage flash drives may be used, possibly including both the "modular" Mac Pro and the iMac Pro.
The improved driver was discovered by various enthusiast communities since the beta High Sierra launch, and first reported by MacObserver. As a result of the improvement, third-party NVMe drives work on compatible motherboards without a hacked driver for the first time.
At present, support for bootable NMVe drives is currently limited to the Hackintosh community as there are no official Macs with a standard NVMe drive slot. In fact, on the vast majority of the motherboards tested by the community, the driver seems to be able to connect to all manufacturers' NVMe drives.
If Apple intended to stick with a custom slot and limited drive support in the future, there would be no need to enhance the NVMe mass storage driver to support the standard slot, found on PC motherboards.
Little is known about the forthcoming "modular" Mac Pro, which was noted to not be shipping in 2017. However, during the WWDC, the iMac Pro was listed has having dual NVMe drives in a RAID configuration to boost speed. It is not clear if the drives in the iMac Pro strictly adhere to the NVMe protocol, or Apple's implementation of it — which is decidedly not standard.
As a side-effect of the driver support, owners of the 5,1 Mac Pro are also seeing support for NVMe drives with a compatible PCI-E adapter, but still cannot boot from them.