usernames are strings, and what exactly is in them doesn't matter.
"1234" is a string in anyone's language. I have to say I've never thought of doing this and can't see a use for it but I'll bet there are some lovely bugs to be discovered. Also, you can have more than one name -> uid. There can be multiple uid = 0 for example, indeed calling uid = 0 "root" is only a convention and is also unnecessary.
I always wonder how bugs (is this a bug?) like that, and that one time when you could log into macOS by repeatedly pressing "Login"[0] even happen. Like, what's the logic behind that?
Reading the issue it looks like Poettering confidently replies that these aren't valid user names, but then when he's challenged to defend that he tries to walk it back to being just not a good idea. So that's how it happens. People make unwarranted assumptions.
Not to mention that it's really confusing if the username is numeric, but the uid != uname. I guess that's why the old debian set uid to uname in this case.
Hey, author here. While I wrote this in 2014 I still take the same steps when setting up a user on a new box, and I haven't had any new problems with it.
Do note I only do this on personal machines - in shared environments or at work I generally use my initials.
"1234" is a string in anyone's language. I have to say I've never thought of doing this and can't see a use for it but I'll bet there are some lovely bugs to be discovered. Also, you can have more than one name -> uid. There can be multiple uid = 0 for example, indeed calling uid = 0 "root" is only a convention and is also unnecessary.