When diet colas first entered the market, beginning with Diet Rite, the Coca-Cola Company had a long-standing policy to use the Coca-Cola name only on its flagship cola, and so its diet cola was named Tab when it was released in 1963. Its rival Pepsi had no such qualms, and after the long-term success of its sugar-free Diet Pepsi (launched in 1964) became clear, Coca-Cola decided to launch a competing sugar-free brand under the Coca-Cola name that could be marketed more easily than Tab. Diet Coke was launched in 1982 and quickly overtook Tab in sales by a wide margin, though the older drink would remain on the market for decades until the COVID-19 pandemic forced Coca-Cola to discontinue Tab along with other slower-selling drinks in 2020.

Diet Coke is not based on the Coca-Cola formula, but instead on Tab. The controversial New Coke, introduced in 1985, used a version of the Diet Coke recipe that contained high-fructose corn syrup and had a slightly different balance of ingredients. In 2005, the company introduced Coca-Cola Zero (renamed Coca-Cola Zero Sugar in 2017), a sugar-free formula more closely based on original Coca-Cola.

In 2005, under pressure from retailer Walmart (which was impressed with the over-the-counter popularity of Splenda sweetener), the company released a new formulation called "Diet Coke sweetened with Splenda". Sucralose and acesulfame potassium replaced aspartame in this version. As the formulation was done to mollify one retailer, this variety had little advertising and promotion, as the company preferred to market Coca-Cola Zero instead. By late 2009, most distributors had stopped distributing the Splenda-formulated Diet Coke.

In 2018, in an effort to be more appealing to millennials, Diet Coke was packaged in a taller, more slender can (of the same volume) and introduced four new flavors. The cans reverted to the conventional shape a year later.
Given this reference text, why and when did Coke use Splenda as a sweetener in diet Coke?
Pressure from Walmart pressured Coca-Cola to release a version of diet Coke sweetened with Splenda from 2005 until most distribution ended in 2009.