1. He gave a clear reason why it is how it is
2. He realizes it is/might be frustrating
3. even `adduser` will not allow it by default
4. The issue that it still runs the unit even with config errors has been addressed: https://github.com/systemd/systemd/commit/bb28e68477a3a39796... (~2 weeks after the issue was opened)
His reason, although clear, is also plainly wrong. Such usernames although bizarre may be encountered by SystemD so it shouldn't break when it sees them. Computer programs, particularly important ones, should be conservative in what they emit and liberal with what they accept and that means not breaking when they encounter weird but technically permissible usernames. His response should have been "Golly, that's a weird username, I didn't think that was possible" and then fix the bug.
There is a certain personality type that likes to reimagine that their original thinking was not flawed, even when presented with a detail that they did not incorporate into their original thinking. If the detail had been in their awareness from the start, they would have arrived at a different position, but they are bound to a strict sense of linearity for reasons inexplicable to me except for ego protection.
Alternatively, if, like he says in the comments of that bug, he really means that SystemD shouldn't support systems that allow such usernames, then he should ensure SystemD won't run on such systems.
Silently doing the wrong thing is not a good thing, especially when "doing the wrong thing" is running stuff as root that wasn't supposed to run as root.
Disclaimer: I know nothing about the particular bug.
Postel's Law has its tradeoffs, and its fuzzy lines are a nice place for security issues to arise.
For sure, there are limits. In this particular case, maybe we say that SystemD shouldn't support weird usernames beginning with numbers, but the other half of the law should still apply. The conservative emission would be logging an error message, not running that unit file as root.
5. useradd does allow it (as noted in a comment). 6. Local users, and the utilities that create them, are not the only source, there things like LDAP and AD.
1. He gave a clear reason why it is how it is 2. He realizes it is/might be frustrating 3. even `adduser` will not allow it by default 4. The issue that it still runs the unit even with config errors has been addressed: https://github.com/systemd/systemd/commit/bb28e68477a3a39796... (~2 weeks after the issue was opened)