Thanks Simon for reviewing this change and correcting my understanding
of Policy on this matter. I agree that would be the best solution. So
postinst/prerm should adduser/deluser.
I wonder how a systemd service file provided by an upstream source
archive is supposed to handle this then, since as I said I proposed a
similar change upstream (which thanks to you I now see should probably
not be merged as-is, though I don't think upstream's current use of
nobody:users is any better, since systemd will even warn about it).