All numeric login names are not supported in AlmaLinux and CentOS. usernames that begin with a digit and also contain letters are not known to have any issue. However, in CentOS 7 version, some system tools can make varying assumptions.

In Almalinux 8 and future releases, ALL numeric usernames are not supported. For more details check the man page of useradd. An excerpt is given below:

Usernames may contain only lower and upper case letters, digits, underscores, or dashes. They can end with a dollar sign. Dashes are not allowed at the beginning of the username. Fully numeric usernames and usernames . or .. are also disallowed. It is not recommended to use
usernames beginning with . character as their home directories will be hidden in the ls output. In regular expression terms: [a-zA-Z0-9_.][a-zA-Z0-9_.-]*[$]?

In CentOS 7, user and group names consisting of only digits, while permitted by shadow-utils,  should best be avoided. OS tools, getent, setfacl, and chown can’t recognize ALL numeric login names. If you still want to create ALL numeric usernames shadow-utils will require the SHADOW_ALLOW_ALL_NUMERIC_USER environment variable to be set to any value so as to allow useradd to make ALL numeric usernames.

POSIX does not forbid ALL numeric user names but it can introduce certain issues. Some programs only manipulate a string or an integer but CLI tools get their input as strings. They have to decide whether 3OO1 should be treated as a string username or an integer UID.