@admitsWrongIfProven it's, um, a systemd joke.

systemd, when it starts a process that will fork a long-lasting daemon + quit, has its own "notify" protocol by which the forking process can inform systemd about the child, so systemd can watch and manage it. that's Type=notify. systemd supports, but disrecommends, "old school" processes that kick out a PID file of the child process too: Type=forking.

the xz hack relied upon sshd getting patched to support "notify".

shoulda stuck with forking.