at the moment this functionality does its best to issue emails. It ignores any WSM options which might in some way stop the email issuing. We could add some functionality which would ignore products if they were no longer in stock. This might result in the email not issuing (if there was only a single item in the basket and that item was no longer in stock).
This issue I see is this: if your item is only temporarily out of stock then you’ll miss an opportunity to get a review. It isn’t always obvious to the web store if an item is temporarily out of stock or gone for good.
No one else as asked for this so it would have to be done a s a custom development. Please open a ticket if you want us to quote you for this.