How to use Back In Stock Notifications for Magento 2
The Back In Stock Notifications extension will help you to enable the stock notifications for guests. Even if customers are not logged-in to their accounts, they'll be able to subscribe to the stock alerts when products are out of stock.
Enable the stocks alerts
Before being able to use the extension, you must first make sure to enable the Allow Alert When Product Comes Back in Stock option from:
Guest Stock Alert Email Template
Once the Stock alert option is enabled, you can select the Stock Alert Email Template (not logged in).