The \Drupal\field_group\FieldGroupFormatterInterface::settingsForm() methods are called in two different contexts:
\Drupal\field_group\Form\FieldGroupAddForm formfield_group_format_settings_form() function called within the field_group_field_ui_display_form_alter() hook implementationIn the edition context, $form and $form_state are passed to the settingsForm method, which is not a problem for PHP that accepts more arguments than defined in method calls.
In the creation context, these arguments are not passed to the settingsForm method.
As the \Drupal\field_group\FieldGroupFormatterInterface::settingsForm() method description does not include these parameters, one specific implementation cannot declare them without throwing a fatal error.
In some specific contrib cases (like UI Patterns Field Group → or SDC Display modules implementations), having these parameters declared would be a good thing, even if your core formatters don't use them.
Add these parameters in \Drupal\field_group\FieldGroupFormatterInterface::settingsForm() method description and all field_group implementations.
Code and documentation.
New parameters on \Drupal\field_group\FieldGroupFormatterInterface::settingsForm().
Interface changes might break existing contrib/custom field group formatters.
This might be the appropriate time to do this before beta1!
Active
4.0
Miscellaneous
You can call me stupid :D
MR and patch updated with the most important part