In Ubuntu/Debian based distributions it has always been possible to allow the use of "." in usernames by modifying the
NAME_REGEX field in
/etc/adduser.conf. The value that I have always found that works is:
I have tried this in elementary OS v7 and it allows me to create a user with a "." in their username using
adduser but the UI doesn't recognise the new user. If I try to add a user using the GUI User Accounts tool it rejects a username with a ".".
It is a while since I used it but I recall this used to work correctly in v6.1. Is the GUI tool using a different configuration file to describe what characters are permitted in a username?