migration ID for 'drush mst' should not be optional

  public function stop($migration_id = '') {
    /** @var \Drupal\migrate\Plugin\MigrationInterface $migration */
    $migration = $this->migrationPluginManager->createInstance(
      $migration_id
    );

There's no reason for this parameter to be optional, as if you omit it, the command can't do anything useful, and you get an error:

 [error]  Migration  does not exist

In MigrateToolsCommands.php line 483:

  Migration  does not exist

(which is confusing if you're misremembering and thinking 'mst' is 'migrate status'!