Updated: Comment #N
When SystemConfigFormBase was added in, we named it directly after system_config_form() and left it in system.module, because there was only \Drupal\Core\Form\FormInterface in that directory.
Now it can live alongside ConfirmFormBase and FormBase
Move \Drupal\system\SystemConfigFormBase to \Drupal\Core\Form\ConfigFormBase
User interface changes
If you extend SystemConfigFormBase, you'll have to fix your use statements.