Re: https://firmwaresecurity.com/2018/07/12/apple-releases-new-systems-with-t2-chip-and-uefi-secureboot/ and

Apple KB article on Secure Boot

the latter Apple support article on Secure Boot has been updated recently:

About Secure Boot

https://support.apple.com/en-us/HT208330

Mac computers that have the Apple T2 chip

https://support.apple.com/en-us/HT208862