As I understand the process, the "resend thank-you email" looks up a transaction ID you provide, determines which Item Numbers were purchased in that transaction, and sends a current thank-you email for those same Item Numbers as if the buyer of record for that old transaction had just purchased those same products right now (including the copy to you, if that's how the product is currently configured). This means it cannot resend emails for deleted products, nor can it send previous versions of the email for a revised product.
May Robin strike me down if I am mistaken. ;^)