Most versions of UNIX provide menu programs to create new users, modify users, retire users, or delete users.
In early UNIX systems, you could not ever delete a user due to security reasons. The fear was that many system tables provide special capabilities by username, not UID number. Thus, if you deleted a user and later added the same name again, that name would still have the old user’s special privileges.
Instead of a delete user option, you sometimes had a retire option. This put the account out of action so that it could not be used, and it also kept the name reserved so that it could not be used for a new user.
Today, it is common to be able to delete a username and later add that username ...