This reminds me of the systemd bug where usernames starting with a digit were mishandled (#15141). It seems to me like something that "should" be relaxed, but

This reminds me of the systemd bug where usernames starting with a digit were mishandled (#15141). It seems to me like something that "should" be relaxed, but we need to have high confidence in the entire foodchain. adduser seems like the last place it should be changed, not the first - anyone requiring "enough rope" is already served by useradd.