Re: https://firmwaresecurity.com/2018/07/12/apple-releases-new-systems-with-t2-chip-and-uefi-secureboot/ and
https://firmwaresecurity.com/2017/12/20/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