Exclude child categories from a category archive page in WordPress

wp-exclude-child-categories.php

<?php
/**
 * Excludes child terms from the main query on the category archive.
 *
 * @param object $query the WP_Query instance
 * @return void
 * @access public
 */
function rk_fix_tax_queries_on_archives( $query ) {

	// Only run this on the main query for the category archive
	if ( ! is_admin() && $query->is_category() && $query->is_main_query() ) {

		// What category is this
		$cat = $query->query_vars['category_name'];

		// Build the new query args
		$tax_query = array(
			array(
				'taxonomy'         => 'category',
				'field'            => 'slug',
				'terms'            => $cat,
				'include_children' => false,
			),
		);

		// Set the new query args to $query->query_vars['tax_query']
		$query->set( 'tax_query', $tax_query );

		// Setting the query args is not enough, we have to create a new tax query object
		// and force feed it to the query
		$query->tax_query = new WP_Tax_Query( $tax_query );

	}
}

add_action( 'parse_tax_query', 'rk_fix_tax_queries_on_archives' );

God bless you, seriously. Thank you so much for this.

Maybe a bit less "force feed it to the query" with the pre_get_posts hook...

This is an example for a custom taxonomy 'localisation' without the need to force a new WP_Tax_Query( $tax_query ); again:

function exclude_children_on_archives( $query ) {
	// Only run this on the main query for a localisation term archive.
	if ( ! is_admin() && $query->is_tax('localisation') && $query->is_main_query() ) {

		// What term is this.
		$term = $query->query_vars['localisation'];

		// Build the new query args.
		$tax_query = array(
			array(
				'taxonomy'            => 'localisation',
				'field'                     => 'slug',
				'terms'                  => $term,
				'include_children' => false
			),
		);

		// Set the new query args to $query->query_vars['tax_query'].
		$query->set( 'tax_query', $tax_query );
	}
}
add_action( 'pre_get_posts', 'exclude_children_on_archives' );