Get all posts without tags

A WP_Query where 'tax_query' has all tag terms and operator ‘NOT IN’:

$tags = get_terms('post_tag', array('fields'=>'ids') );
$args = array(
  'post_type' => 'post',
  'posts_per_page' => -1,
  'tax_query' => array(
    array(
      'taxonomy' => 'post_tag',
      'field' => 'id',
      'terms' => $tags,
      'operator' => 'NOT IN'
    )
  )
);
$untagged = new WP_Query( $args );