The PHP extension mssql was removed in PHP 7.0. As Moodle requires at least 7.0 (as of Moodle 3.4), use of Microsoft SQL Server databases is now only possible using the sqlsrv PHP extension. Consequently code using the mssql extension, i.e. resulting in calls to the removed mssql_XXX() functions should be removed as 1) the mssql and mssql_n database types are still listed for external database authentication and enrolment even though they cannot work causing confusion and 2) this is redundant code and will become unmaintained (which may lead to security vulnerabilities in the future).