While I'll ignore the System D hyperbole, your point about Unix has merit.

I think the *BSD are also good, at least from an educational standpoint, with their relative simplicity and low system requirements. Since there is a lot of integration making a from scratch distro might take less material, but it could be supplemented with more in depth/sysadmin exploration.

From an education standpoint for those who really, really want to understand, the *BSD init and SysVinit systems require direct human administration. You break it, you fix it. Then, and only then, does learning systemd's ''then something happens behind the curtain'' type of automation make sense. If the student decides that one is more suitable than the other(s), they've done so from an enlightened vantage point.