Customize the WooCommerce Email Template Subject for Wholesale Customers

/**

 * Customize the Subject for wholesalers

 */

add_filter( 'woocommerce_email_subject_customer_completed_order', function( $subject, $order ) {

    if ( ! $order ) return $subject;

    $customer_id = $order->get_customer_id();

    if ( ! $customer_id ) return $subject;

    $wholesale_roles = [ 'wholesale_customer', 'wholesale_vip' ];

    $user = get_userdata( $customer_id );

    if ( $user && array_intersect( $wholesale_roles, (array) $user->roles ) ) {

        // We use get_bloginfo('name') instead of {site_title}

        return 'Your wholesale order with ' . get_bloginfo( 'name' ) . ' is complete';

    }

    return $subject;

}, 10, 2 );