Trivial in retrospect. There were several ssh* files in /lib/systemd/service. None named sshd*. I copied the one named ssh.service to sshd.service, enabled it, rebooted, and there is was.
An interesting question is why things are different in Raspian Stretch on a 3+. For the time being, I'll just blame the RPi4 OS/motherboard. Or me. Thanks all. -- Glenn English
