Ah, thanks!
Ran it on the 2012 11", which had a clean install of High Sierra after the SSD change and it has it. It actually says it has four volumes:
disk1s1 (No specific role)
disk1s2 (Preboot)
disk1s3 (Recovery)
disk1s4 (VM)
Currently reinstalling High Sierra on the 2015 11" (I hope you can still follow which is which, this is the latest one, the “successor”! 
) after entering recovery mode with cmd-R. I hadn’t checked if there was a recovery volume in disk utility, but it didn’t go into internet recovery so I suppose there was). I’ll run the command in Terminal again when it’s done.
No experiments on the 13", which is itself a clone, so it may not have it, since that’s my main machine for the moment, until I’m fully satisfied that the “new” 2015 11" is working without fault. Then maybe I’ll trade in one (possibly two) of the remaining machines.
update
High Sierra reinstalled, recovery partition is there.
It must have been there in the clone too, I think that used to be a problem in previous versions of SuperDuper.