There’s a currentlyNotInStock boolean on the ProductVariant:
currentlyNotInStock (Boolean!)> Whether a product is out of stock but still available for purchase (used for backorders).
I’d assume you’d need to use this in combination with availableForSale and quantityAvailable:
availableForSale (Boolean!)> Indicates if the product variant is available for sale.> > quantityAvailable (Int)> The total sellable quantity of the variant for online sales channels.
There’s an additional permission: unauthenticated_read_product_inventory access inventory. You’ll need to configure this from your private app in order to get anything useful for quantityAvailable.
https://shopify.dev/docs/storefront-api/reference/products/productvariant#fields-2021-01
You may also want to still add additional tag, collection, product type or meta field to make the behaviour explicit opt in.