So I tested xinetd/systemd socket use on a nice, clean container starting from a clean configuration.
Tests with Ubuntu 20.04.3 LTS
No xinetd installed, install agent 1.6.0p25 -> systemd socket with warning
xinetd installed, installed agent 1.6.0p25 -> xinetd
xinetd installed, installed agent 1.6.0p25 and installed 2.0.0p12 over first -> converted to systemd socket, config file removed from /etc/xinetd.d
xinetd installed, installed agent 2.0.0p12 -> systemd socket
So it looks like this is deliberate and I’m a bit sad, as it obviously has some problems that I can force to go away by manually reverting to using xinetd with the 2.0.0p12 agent.
I’ll quietly ignore that on some of my production hosts the xinetd configuration wasn’t actually fully cleaned up…
At least I’m no longer sitting here thinking I was careless and left that many hosts without properly setup xinetd.
Thanks everyone for your input.