Google has added support to mark a product as back ordered for its product rich results. You can now set the availability property with a BackOrder value to communicate to Google Search that the item is back ordered.
Google added this new value to the recommended properties availability section of the product structured data developer document. The BackOrder value indicates that the item is available on back order. Google also supports these availability values for rich results Discontinued, InStock, InStoreOnly, LimitedAvailability, OnlineOnly, OutOfStock, PreOrder, PreSale and SoldOut. But BackOrder is the new item that was added yesterday.
Here is what product structured data can look like in search, when Google adds the rich results to the snippet:
I assume when Google picks up on the BackOrder value, it will show that to the searcher in the search results or maybe Google just won’t show back ordered items in search?
Forum discussion at Twitter.