In actuality, Pepsi Throwback is not a diet soda of any kind. It does not contain saccharine, aspartame, or sucralose.
The only difference between Pepsi Throwback and Pepsi is that Pepsi is made using HFCS-55 as a sugar substitute. HFCS-55 is the most widely used version of high-fructose corn syrup in the United States and Canada and is linked by various medical professionals to obesity, hyperglycemia, and type-II diabetes.
Pepsi Throwback, on the other hand, simply replaces HFCS-55 with regular 'ol sugar. The name is derived from the fact that the original recipe of Pepsi, changed little until the early 1980's in the US, used more expensive cane sugar as a sweetener. Fans of Pepsi often enjoy this sugar formulation far more than its HFCS-55 substitute as sugar is said to give the drink more "snap" and less rebound thirstiness.
Since Pepsi Throwback was a limited-release beverage, released to coincide with the NFL Super Bowl in 2010, it is no longer distributed. "Leftover" cases can sometimes be found at Costco or other wholesale stores.
However, hard-core Pepsi aficionados know that they need only wait until Passover season to purchase Pepsi made with sugar instead of HFCS-55 since the CRC does not consider HFCS-55 a kosher ingredient. "Passover Pepsi" is (usually) easily identifiable by the bottler using yellow caps (on the 2l bottles) that have a K with a circle surrounding it printed on their tops. This author knows that the Coca-Cola Company uses the same procedure to identify Coke made with kosher sugar on their 2l bottles--yet has no idea if either Pepsi or Coca-Cola is available in its kosher (HFCS-55-free) form in 12oz cans...or how these are identified if so.