// get all paragraphs
$paragraph_ids = Drupal::entityQuery('paragraph')->execute();
$paragraphs = Drupal::entityTypeManager()->getStorage('paragraph')->loadMultiple($paragraph_ids);
foreach ($paragraphs as $target_id => $paragraph) {
  // get parent entity (node, taxonomy, paragraph, etc.)
  $parent = $paragraph->getParentEntity();
  $field_name = $paragraph->parent_field_name->value;

  // Check if current paragraph exists in parent entity field values
  $exists = FALSE;
  $values = $parent->get($field_name)->getValue();
  foreach($values as $value) {
    if ($value['target_id'] == $target_id) {
      $exists = TRUE;
    }
  }

  // Delete paragraphs that aren't linked to an entity they claim as a parent
  if (!$exists) {
    $paragraph->delete();
    var_dump($target_id);
  }
}