Exclude ALL posts from sub categories

exclude-subs.php

<?php

// https://wordpress.stackexchange.com/questions/64701/exclude-all-posts-from-sub-categories

function filter_out_children( $query ) {
  if ( is_main_query() && is_category() && ! is_admin() ) {
     $qo = $query->get_queried_object();
     $tax_query = array(
       'taxonomy' => 'category',
       'field' => 'id',
       'terms' => $qo->term_id,
       'include_children' => false
     );
     $query->set( 'tax_query', array($tax_query) );
  }
}
add_action('pre_get_posts', 'filter_out_children');