I personally like to use the "Now Back In Stock", when a customer asks you that you just add their email on the product for notification, once it goes back in stock it will trigger an automatic notification to the customer. You can also use SMSs should you like as well!
When a product is back in the store, you will have to notify customers back in stock email alerts manually. Shopify or Out of Sandbox Shopify themes doesn’t support automatic back in stock/restock alerts.
However, you can automate the stock notifications with the AppJetty CommerceXpand app. It consists of a Back in Stock Alerts app that automatically informs the customers about the product they abandoned due to product unavailability.
In addition, the app consists of different small apps like Abandonment Cart Manager, Sticky Add to Cart, Quick Buy Button, and more.
You can find CommerceXpand on the Shopify App Store.