How to customize the subject of a Commerce order confirmation e-mail? Here you can learn how to customize the content of the order confirmation mail, but what about customize the subject of the mail?
In a custom module you can try something like:

/**
 * Implements hook_mail_alter().
 */
function mymodule_mail_alter(array &amp;$message) {
  // Order receipt.
  if ($message['id'] === 'commerce_order_receipt') {
    /** @var \Drupal\commerce_order\Entity\OrderInterface $order */
    $order = $message['params']['order'];

    // Change the email subject.
    // @todo Remove this if/when it becomes configurable.
    //   https://www.drupal.org/project/commerce/issues/2924159
    $message['subject'] = t('Some other email subject #@number', [
      '@number' =&gt; $order-&gt;getOrderNumber(),
    ]);
  }
}