Apple has clarified the Secure Boot function of the iMac Pro’s embedded T2 chip:
Full Security is the default Secure Boot setting, offering the highest level of security. This is a level of security previously available only on iOS devices.
During startup when Medium Security is turned on, your Mac verifies the OS on your startup disk only by making sure that it has been properly signed by Apple (macOS) or Microsoft (Windows). This doesn’t require an Internet connection or updated integrity information from Apple, so it doesn’t prevent your Mac from using an OS that is no longer trusted by Apple.
The No Security setting doesn’t enforce any of the above security requirements for your startup disk.
Glad to see this coming to the Mac, but I’m glad to see that users who need to turn it off can do so.