WooCommerce - Add additional taxonomies to product related posts

functions.php

add_filter( 'woocommerce_product_related_posts_query', 'product_related_posts_query' );

function product_related_posts_query( $query ) {
	global $product, $wpdb;
	
	if ( is_object( $product ) ) {
		$terms_array = array(0);

		// get brands
		$terms = wp_get_post_terms( $product->id, 'product_brand' );

		if ( ! isset( $terms ) ) {
			return $query;
		}

		foreach ( $terms as $term ) {
			$terms_array[] = $term->term_id;
		}

		$brands_array = array_map( 'absint', $terms_array );

		if ( sizeof( $brands_array ) == 0 ) {
			return $query;
		}
		
		// optionaly hide out of stock products
		$query['join'] .= " INNER JOIN {$wpdb->postmeta} pm2 ON ( pm2.post_id = p.ID AND pm2.meta_key='_stock_status' )";

		$exclude_ids = array_map( 'absint', array_merge( array( 0, $product->id ), $product->get_upsells() ) );

		$query['where'] .= " OR ( ( tt.taxonomy = 'product_brand' AND t.term_id IN ( " . implode( ',', $brands_array ) . " ) )";
		$query['where'] .= " AND p.ID NOT IN ( " . implode( ',', $exclude_ids ) . " ) )";

		// optionaly hide out of stock products
		$query = str_replace( "AND pm.meta_value IN ( 'visible', 'catalog' )", "AND pm.meta_value IN ( 'visible', 'catalog' ) AND pm2.meta_value = 'instock'", $query );
	}

	return $query;
}

I like this, and it works, but sadly only if you have a product_cat term or a product tag set. woocommerce never gets to running the woocommerce_product_related_posts_query filter if there is no default woocommerce categories or tags set. It isn't an issue with this code, just a note that if you find the code not working that could be the reason.

Your right, it works on the assumption that default categories have already been added or set up.