ICYMI: we've recently done some work on the #systemd documentation rendered on freedesktop.org, and now all manpages have a drop-down menu to select the release version to visualize. Also individual options are now tagged with the version they were first introduced in. These improvements should hopefully help readers with understanding what options are available in what version.
Example: https://www.freedesktop.org/software/systemd/man/255/systemd.exec.html