...
For this reason, you will see a Warning Message saying Friendly is reserved if you try to
sign up as a user using those namesa reserved name
rename your own user account to have those names a reserved name
rename an Organisation-controlled User to have one of those names.a reserved name
Your user names should be the real names of people - apart from the exceptional cases where a person’s real name includes a reserved word, such as Admin or Edit.
...
If you are renaming users as part of the deactivation process and you do not want to use the real name of the user: Rename , rename the user to an alternative name e.g.
“z - not in use - 1”,
“z - not in use - 2”
etc.