Form classes are normally not supposed to be extended by other modules. That holds true for the `\Drupal\system_account\Form\SystemAccountSettingsForm` class. If modules need to alter the form it implements, they can implement a hook. In the 2.0.x branch, the class should be marked `@internal`; in the 3.0.x branch, the class should be marked `final`. > Change record: [#3600593](https://www.drupal.org/node/3600593)