User=0day considered harmful in systemd

> One can argue that systemd should not be checking usernames for itself

Not sure if that's the best criticism, I don't think it makes sense. But one could argue that how things are handled are suboptimal either way. First introducing somewhat arbitrary restrictions, then silently ignoring them and default back to root.
But what's the case for it?

> But systemd is rather opinionated about how things should work, naming, and so forth,

Ok... ? The sentence starts with "but", yet there's no argument made. They are 'opinionated about something they shouldn't be? That doesn't really say anyhting.

> so it should not come as an enormous surprise that it enforces its rules on system usernames.

Well, I guess it did come as a surprise, hence the discussion.

> As Poettering pointed out—several times—systemd is free software; users, distributions, and others are all welcome to modify it to suit their needs.

"If you don't like it do it better!" the universal last line of defense. Of course that's true. People can adjust it. They could patch systemd. It isn't an argument for handling it the way it's done in the first place, though.