Current IPA environment is using lowercase usernames.
But we also have a LDAP environment in which usernames are in UPPERCASE. This is used for "some" krb tickets possibilities.
Imagine we add users to the Default Trust View and adapt login to UPPERCASE. Can we expect some troubles or nuisances (think SSO, SSH, Kerberos...)
I remember very good some installers don't like adding local users on installation with a login in uppercase.
We can use other override(s), but maybe this is not feasible from a user management perspective.
Especially regarding the normal typical POSIX way, which was also described in the username restrictions thread (10/2018)