I suspect that systemd people are looking at this thread in perplexity, and probably doing their thing (that I've seen over the years) of regarding the world of Debian as being amazingly behind the times in places.

The SSH server being a socket unit with systemd doing all of the socket parallelism-limiting and accepting was one of the earliest examples of socket activation ever given in systemd. It was in one of Lennart Poettering's earliest writings on the subject back in 2011.

* https://0pointer.de/blog/projects/inetd.html

And even that wasn't the earliest discussion of this way of running SSH by a long shot, as this was old news even before systemd was invented. One can go back years earlier than even Barrett's, Silverman's, and Byrnes's SSH: The Secure Shell: The Definitive Guide published in 2005, which is one of many places explaining that lots of options in sshd_config get ignored when SSH is started by something else that does all of the socket stuff.

Like inetd.

This has been the case ever since it has been possible to put an SSH server together with inetd in "nowait" mode. Some enterprising computer historian might one day find out when the earliest mention of this was. It might even be in the original 1990s INSTALL file for SSH.