Programmatically populate the “Sale” product category with products on sale

code.php

<?php

/**
 * Based on https://www.businessbloomer.com/woocommerce-sale-category-automatic/
*/
add_action( 'pre_get_posts', 'luizpb_wc_sale_category' );
function luizpb_wc_sale_category ( $query ) {
	$term_slug = 'sale';
	if ( $query->get( 'product_cat' ) !== $term_slug ) return;
	$query->set( 'product_cat', null );
	$query->set( 'post_type', 'product' );
	$query->set( 'post__in', wc_get_product_ids_on_sale() );
}