Out of Stock Redirect
Redirect to a related product when a product is out of stock
When is feature is enabled, if a product is no longer available (out of stock in all variants), the user will be redirected to the URL that you specify.
In most cases, it's a good idea to redirect to the collection page that the product belonged to, or a similar item.
When more stock is added to the Shopify admin panel, the redirect will stop working and the product page will be shown.