From what I remember, the
Double Decker Taco was when
Taco Bell started getting weird with their menu selections. Now I don't mean that in a bad way and it might not be true but, in my mind, before the Double Decker Taco, Taco Bell was tacos, burritos, nachos, etc. You know, traditional Mexican-themed dishes.
After the Double Decker, they started coming out with things like the Crunchwrap Supreme, the Chalupa, the Gordita, etc. Things that are either completely Taco Bell or so very far from their original origins that they are still like de facto Taco Bell.