Currently, it is used in in AuthPluginPrimaryAuthenticationProviderTest.php but no performer is specified, therefore when Notifications is enabled the test fails with this error message:
1) MediaWiki\Auth\AuthPluginPrimaryAuthenticationProviderTest::testOnUserGroupsChanged Missing argument 4 for EchoHooks::onUserGroupsChanged(), called in /var/www/html/includes/Hooks.php on line 177 and defined /var/www/html/extensions/Echo/Hooks.php:703 /var/www/html/includes/Hooks.php:177 /var/www/html/includes/Hooks.php:205 /var/www/html/tests/phpunit/includes/auth/AuthPluginPrimaryAuthenticationProviderTest.php:76 /var/www/html/tests/phpunit/MediaWikiTestCase.php:416 /var/www/html/maintenance/doMaintenance.php:95
Of note, if the performer is provided, then you get an error for "Missing argument 5" (i.e. reason must also be provided). A default value for $reason should be added here.