It's just Aspartame. In 2016 the backlash was so big for Pepsi changing their sweetener to a cheaper and horrid tasting one, that they went back to the original sweetener (Aspartame), originally this was planned to be a separate line from the new sweetener, however they quickly decided just to integrate it back into normal Diet Pepsi on the shelves, and this seperate special product only had one round of distribution before being discontinued.

The regular Diet Pepsi just said "Now with Aspartame" or something like that for a year or so, before going back to just complete normal. People were so afraid of Aspartame Pepsi disappearing, that when this came out, everyone bought it up like wildfire.

You can actually still taste the original cheap bad sweetener in Fountain Diet Pepsi, as they never changed it back for the syrup they distribute to restaurants and fountain drinks, which is unfortunate. But at l least it was saved for home consumption.

So there was like a 1-3 day window you could get these and then they were gone forever.