....preorder (preorder) or backorder (backorder) to indicate that the product is not currently in stock, but will be in the future. In order to do this, you must also provide an availability date using the availability date [[availability_date]
This could be expanded....but it requires per-site customisation for the out of stock options, per manufacturer/category...
There are multiple options: https://developers.google.com/search/docs/appearance/structured-data/product
Currently code outputs in stock : https://schema.org/InStock: The item is in stock. out of stock: https://schema.org/PreOrder: The item is available for pre-order.
But...PreOrder needs a date.
This could be expanded....but it requires per-site customisation for the out of stock options, per manufacturer/category...