I have the dealer brochures for the 1980 and 1981 Miradas, and they are listed under options. I don't know if they dealer brochures are showing something that wasn't actually available, or if it was a rarely ordered option. In the 3 J body cars that I have owned, none of them had the inflatable spare. The trunk floor of my 1981 Mirada does have a well at the bottom of the trunk that is too small for a standard 15" spare. I take it that it was intended for the inflatable spare, but the actual spare (which I believe was factory) is full sized and in the standard position. The standard spare was factory, as the hold down hardware for the standard spare is all there.
The spare in a Mirada or Cordoba sits in the middle of the trunk, taking up a lot of space. A "skinny" spare, whether the inflatable with a bottle type, or the newer "temporary" spare can be put into one of the "wells" that those cars have on the sides of the trunk space, freeing up a ton of space.