$orderby_value = isset($_GET['orderby']) ? wc_clean($_GET['orderby']) : apply_filters('woocommerce_default_catalog_orderby', get_option('woocommerce_default_catalog_orderby'));

if ('featured' == $orderby_value) {

    $args['orderby'] = "terms";
    $args['order'] = 'DESC';
    $args['tax_query'] = array(
        array(
            'taxonomy' => 'product_visibility',
            'field'    => 'name',
            'terms'    => 'featured',
            'operator' => 'IN',
        ),
    );

}


return $args;

add_filter('woocommerce_catalog_orderby', 'wc_catalog_orderby');

function wc_catalog_orderby($sortby)
{
    $sortby['featured'] = 'Sort by featured';
    return $sortby;
}