Comment 11 for bug 1993478

Port
Specifies the port number that sshd(8) listens on. The default is 22. Multiple options of this type are permitted. See also ListenAddress. Note: On Ubuntu, the openssh-server package is configured to use systemd socket-based activation by default. Therefore if you are using systemd with the default configuration, Port options will not be honored. Address configuration must be handled in /etc/systemd/system/ssh.socket.d instead.

# Port and ListenAddress options are not used when sshd is socket-activated,
# which is now the default in Ubuntu. See sshd_config(5) and
# /usr/share/doc/openssh-server/README.Debian.gz for details.
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For me, the post upgrade error didn't happen on the machine that had the default openssh-server port 22, but on the machine that had a custom port and bind address. I had to purge and reinstall openssh-server to fix the broken, partially installed, package post-upgrade.

I only noticed the new message in the sshd_config configuration file after purging/reinstalling openssh-server second time on one of my machine. I was multitasking during the upgrade process, if there was a message about the new port configuration being a systemd socket, I missed it.