Per https://www.php.net/manual/en/mysql.installation.php ,

This extension was deprecated in PHP 5.5.0, and it was removed in PHP 7.0.0.

Phorge requires PHP 7.2 since T15047.

Thus checks like the second one of if (!extension_loaded('mysqli') && !extension_loaded('mysql')) in
https://we.phorge.it/source/phorge/browse/master/src/applications/config/check/PhabricatorExtensionsSetupCheck.php can be removed, and entire https://we.phorge.it/source/phorge/browse/master/src/infrastructure/storage/connection/mysql/AphrontMySQLDatabaseConnection.php (run arc liberate after!) being called from https://we.phorge.it/source/phorge/browse/master/src/infrastructure/cluster/PhabricatorDatabaseRef.php).

Unfortunately that extension name is a bit generic to grep but this should be it.