The other user data is already stored in the userdb versions that nearly everyone who uses a systemd distro already has. You can check what data is being stored with userdbctl. On my system that looks like this:

userdbctl user --output=json $(whoami)
{
        "userName" : "sky",
        "uid" : 1000,
        "gid" : 100,
        "homeDirectory" : "/users/sky/home"
        "shell" : "/run/current-system/sw/bin/fish"
}

Honestly this PR is a bit of a nothingburger. I'm not aware of any distro really using userdb to store data beyond what you'd store in /etc/passwd (maybe Ubuntu does?). The main value of userdb seems to be as a frontend so other programs don't need dedicated code to handle /etc/passwd, LDAP, etc. Notably GNOME recently eliminated their dedicated code in favor of just using userdb.

And Userdb doesn't really handle data validation at all. It enforces that you input a valid date after 1900, but that's kinda it. I guess you need root/sudo privileges to change the birthdate but that's not much of a hurdle for most Linux users.

Really this entire PR boils down to:

ALTER TABLE users ADD birthdate date;