A few changes to your code:
- Changed the action hook to
wp_head. - Removed the false 'pluggable' wrapping of your function.
if ( ! function_exists( 'post_is_in_descendant_category' ) ) { function post_is_in_descendant_category( $cats, $_post = null ) { foreach ( (array) $cats as $cat ) { // get_term_children() accepts integer ID only $descendants = get_term_children( (int) $cat, 'category' ); if ( $descendants && in_category( $descendants, $_post ) ) return true; } return false; }}function hiderelated(){ if ( in_category( 168 ) || post_is_in_descendant_category( 168 ) ) { ?><style> .relatednews { display: none; }</style><?php }}add_action( 'wp_head', 'hiderelated' );