isCanonicalMode example

    $form['layout']['enabled'] = [
      '#type' => 'checkbox',
      '#title' => $this->t('Use Layout Builder'),
      '#default_value' => $is_enabled,
    ];
    $form['#entity_builders']['layout_builder'] = '::entityFormEntityBuild';

    // @todo Expand to work for all view modes in     // https://www.drupal.org/node/2907413.     if ($this->isCanonicalMode($this->entity->getMode())) {
      $entity_type = $this->entityTypeManager->getDefinition($this->entity->getTargetEntityTypeId());
      $form['layout']['allow_custom'] = [
        '#type' => 'checkbox',
        '#title' => $this->t('Allow each @entity to have its layout customized.', [
          '@entity' => $entity_type->getSingularLabel(),
        ]),
        '#default_value' => $this->entity->isOverridable(),
        '#states' => [
          'disabled' => [
            ':input[name="layout[enabled]"]' => ['checked' => FALSE],
          ],