entity

/**
* Implements hook_entity_type_alter().
*/
function MYMODULE_entity_type_alter(array &$entity_types) {
 if (isset($entity_types['image_style'])) {
   $entity_types['image_style']->setClass(CustomImageStyle::class);
 }
}

$tags = $form_state->getValue('tags');
$flat_tags = [];
array_walk_recursive($tags, static function ($a) use (&$flat_tags) {
  $flat_tags[] = $a;
});

// Or simple:

$flat_tags = array_column($tags, 'target_id');

   $content_entity_types = [];
   $entity_type_definations = \Drupal::entityTypeManager()->getDefinitions();
   /* @var $definition EntityTypeInterface */
   foreach ($entity_type_definations as $definition) {
     if ($definition instanceof ContentEntityType) {
       $content_entity_types[] = $definition;
     }
   }