If a physical product is out of stock, customers can create and request notification once they desired item is back in stock.
In the customer’s account page, there is a tab for notifications, where they can see the products they want to be notified when it’s back in stock. They can also delete it if they don't want to be notified anymore.
When you update the stock of a physical product, everybody who wishes to be notified for its availability will receive an email that the item is already back in stock.