Google added a support to its product structured data to mark a single product as back ordered. Users can set the property of product availability with back order value to connect with google search that the item is back ordered.

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Users can get this value at the recommended properties availability section of the product structured data developer document.

Google also provide these availability values for rich results: Discontinued, InStock, InStoreOnly, LimitedAvailability, OnlineOnly, OutOfStock, PreOrder, PreSale and SoldOut.

Checkout How Product Structured Data can Look Like in Search: