<?php
/*
* Best way to remove out of stack products from related products
*/
add_filter('woocommerce_product_related_posts_query','aah_product_related_posts_query', 10, 2);
function aah_product_related_posts_query( $query, $product_id ) {

	global $wpdb;
	$query['join']  .= " LEFT JOIN wp_postmeta ON ( p.ID = wp_postmeta.post_id )";
	$query['where']  .= " AND ( wp_postmeta.meta_key = '_stock_status' AND wp_postmeta.meta_value = 'instock' )";

	return $query;
}