Switch from Intel to their own ARM CPUs will hit the Hackintosh community really hard.
The could have done this, if they wanted to, with their own motherboard.
With the T2 chip which is already present in other Macs. It’s optional at the moment but I assume at some point it will be closed off. And then there’s the ARM CPU argument - we could get those soon.
According to a Linus Tech Tips video[1] that I saw a a couple of days ago, you can use PCI nvme drives on the new Mac Pro, but it won't even boot unless one of the proprietary Apple SSDs is connected (because of the T2).
You should be able to turn off the T2 function Secure Boot which blocks booting from other drives: https://support.apple.com/en-us/HT208330
According to this newer Linus Video [1], one of the T2 SSD slots still needs to be populated with an Apple drive to boot from other drives.
Which makes sense: it needs to load firmware/EFI from somewhere; and in a setup where you expect your own SSD to be a fixed resource in the system, there's little reason to have separate BIOS flash.
New Macintoshes come with an Apple-produced "security chip", which is an ARM like those found in iPhones and iPads. They communicate with the main CPU and provide it the boot code via SPI for security, but the security chip itself has an entirely cryptographically signed boot chain and will only boot Apple-provided code.
The presumption is that when enough time has passed that Apple will obsolete the non-security-chip-equipped macs and macOS will only boot when able to communicate with a security chip running Apple-signed code. It's quite plausible. The security chip does provide a ton of other benefits, though.
Any info on how they will lock it down?