src/Security/Security.php
public static function setDefaultAdmin($username, $password)
    {
        Deprecation::notice('5.0.0', 'Please use DefaultAdminService::setDefaultAdmin($username, $password)');

        DefaultAdminService::setDefaultAdmin($username, $password);
        return true;
    }
public static function setDefaultAdmin($username, $password)
    {
        Deprecation::notice('5.0.0', 'Please use DefaultAdminService::setDefaultAdmin($username, $password)');

        DefaultAdminService::setDefaultAdmin($username, $password);
        return true;
    }
public static function setDefaultAdmin ( $ username , $ password ) { Deprecation :: notice ( '5.0.0' , 'Please use DefaultAdminService::setDefaultAdmin($username, $password)' ) ; DefaultAdminService :: setDefaultAdmin ( $ username , $ password ) ; return true ; }
Set a default admin in dev-mode

This will set a static default-admin which is not existing
as a database-record. By this workaround we can test pages in dev-mode
with a unified login. Submitted login-credentials are first checked
against this static information in {@link Security::authenticate()}.

@param string $username The user name
@param string $password The password (in cleartext)
@return bool True if successfully set

@deprecated 4.0.0:5.0.0 Please use DefaultAdminService::setDefaultAdmin($username, $password)
Set a default admin in dev - mode