According to
this Apple support article ⌘S to boot into single user mode no longer works but you can still boot into single user mode via the Recovery drive (⌘R) and this caused by the T2 chip, NOT Mojave.
Although command-S booting into single-user mode either is or will eventually be a thing of the past, its functionality is (in answer to my question) available, but
not duplicated, in Recovery, via Disk Utility and Terminal.
But I
think the new functionality is enhanced in that it can now read/write to ANY volume that appears in your DU sidebar whereas S-UM could access only your boot drive?
According to
Mac startup key combinations Command-S: Start up in single-user mode. This key combination requires macOS High Sierra or earlier. (Emphasis added)
(hence my "beginning with macOS 10.14, Mojave"), but that's incorrect; I was able to command-S boot into S-UM in macOS 10.14.2, Mojave, on my NON-T2 MBP.
I
think I"m correct in assuming that the change
couldn't be attributable to Mojave, because S-UM is firmware functionality which can't be affected by a change in the OS?
I also booted into Recovery to see if Disk Utility and Terminal function as I've read they do with a T2 chip, and I found that they DO NOT.
So the T2 is, indeed, THE determining factor.