All classes that implement FormStateInterface in Drupal 9
- class \Drupal\Core\Form\FormState implements FormStateInterface uses FormStateValuesTrait
- class \Drupal\Core\Form\FormStateDecoratorBase implements FormStateInterface
- class \Drupal\Core\Form\SubformState implements SubformStateInterface uses FormStateValuesTrait
- class \Drupal\Tests\Core\Form\NonAbstractFormStateDecoratorBase