/**
 * Show out of stock variation selections as greyed out
 */

add_filter('woocommerce_variation_is_active', 'cc_out_of_stock_variations_show_grey', 10, 2);

function cc_out_of_stock_variations_show_grey($is_active, $variation) {

  if (!$variation->is_in_stock()) return false;

  return $is_active;

}