/**
 * @snippet       Sort Products By Stock Status - WooCommerce Shop
 * @how-to        Get CustomizeWoo.com FREE
 * @author        Rodolfo Melogli
 * @compatible    WooCommerce 3.9
 * @donate $9     https://businessbloomer.com/bloomer-armada/
 */
 
add_filter( 'woocommerce_get_catalog_ordering_args', 'bbloomer_first_sort_by_stock_amount', 9999 );
 
function bbloomer_first_sort_by_stock_amount( $args ) {
   $args['orderby'] = 'meta_value';
   $args['order'] = 'ASC';
   $args['meta_key'] = '_stock_status';
   return $args;
}