Magento – How to Add Custom Backorder Message

To add it to the Email:

app/design/frontend/default/<your_template>/template/email/order/items/order/default.phtml

and find this ~line 31:

<strong style="font-size:11px;"><?php echo $this->htmlEscape($_item->getName()) ?></strong>

Add the following line right after that:

    <?php if ($_item->getQtyBackordered()): ?>

        <p style="font-size: 10px;color: #CCC">
   * This product is not available  in the requested quantity. <?php echo  
   $_item->getQtyBackordered()*1 ?> of the items will be backordered.</p>

    <?php endif; ?>

HTML is not allowed by default in flash messages and it's escaped. To change this you can use addSuccess() method instead of addSuccessMessage() to create message.

It can be done by creating plugin forMagento\Checkout\Controller\Cart\Add controller. I would avoid overwriting whole execute() method only for this small change and rather use the following:

public function afterExecute()
{
    $lastAddedMessage = $this->messageManager->getMessages()->getLastAddedMessage();
    if ($lastAddedMessage && $lastAddedMessage->getType() == MessageInterface::TYPE_SUCCESS) {
        $text = $lastAddedMessage->getText();

        /**
         * @var \Magento\Framework\Message\Collection
         */
        $messages = $this->messageManager->getMessages();
        $messages->deleteMessageByIdentifier($lastAddedMessage->getIdentifier());

        $cartUrl = $this->_objectManager->get('Magento\Checkout\Helper\Cart')->getCartUrl();
        $this->messageManager->addSuccess($text . '<a href="'.$cartUrl.'">View cart</a>');
    }

    return $this->goBack();
}