Magento 2 Out of Stock Notifications
Override the default add to cart for products with an out of stock notifcation popup, allowing a customer to subscribe interest in the product, this is viewable in the admin under Product > Out Of Stock Notifications. The customer will then be issued a subscribed email, and when the product is back in stock the cron will run at midnight sending a back in stock email if the product is back in stock and removing the notification from the admin.
This is not a supported module.