code: public static boolean isIdOrFullnameAllowed(@CheckForNull String id) {
          if (StringUtils.isBlank(id)) {
              return false;
          }
          final String trimmedId = id.trim();
          for (String invalidId : ILLEGAL_PERSISTED_USERNAMES) {
              if (trimmedId.equalsIgnoreCase(invalidId))
                  return false;
          }
          return true;
      }
docs: Is the ID allowed? Some are prohibited for security reasons. See SECURITY-166.
  <p>
  Note that this is only enforced when saving. These users are often created
  via the constructor (and even listed on /asynchPeople), but our goal is to
  prevent anyone from logging in as these users. Therefore, we prevent
  saving a User with one of these ids.
  
  @param id ID to be checked
  @return {@code true} if the username or fullname is valid.
  For {@code null} or blank IDs returns {@code false}.
  @since 1.600