"Godspeed," episode 18 of season 5 of The Flash, made mention of a Purple Speedster whose identity was unknown to the people of Central City 2049. While this may seem like a new mystery for viewers to ponder, the truth is far closer at hand than most would suspect - particularly Nora-West Allen.
Most of "Godspeed" is devoted to a flashback detailing Nora West-Allen's superhero origin story and how she discovered her heroic legacy and superpowers while trying to catch the titular speedster criminal. Nora first uncovered evidence of Godspeed's existence while investigating the robbery of a chemical transport vehicle and pondering the dual mysteries of how the thief wasn't caught on camera and was able to steal anything before the transport was filled with knock-out gas. The only answer that made any sense based on all the available evidence was that the thief was a metahuman with super-speed powers. It was a theory that Nora seemingly confirmed after slowing down the video footage and detecting a tell-tale streak of white lightning moving around the truck's interior.
The white lightning raised a new conundrum for Nora and her co-worker, Lia, who were both superhero trivia buffs. They both knew that no speedster in Central City had ever manifested white lightning when they moved - a fact Nora later confirmed with a visit to the Flash Museum, which only recorded yellow, red, blue and purple lightning manifesting in the past. As Lia pondered this, she ran through a list of known speedsters, including The Flash, Kid Flash, Jay Garrick and "that weird purple speedster that just showed up one day." It would be easy to think this was a hint toward Nora's eventually destiny, to go back into the past and assist her father as the hero XS.
Predestination in The Flash is rarely that easy, however, and the show has largely avoided playing with the old time-travel game where the hero figures out what they have to do based on a hint of how they already must have done it using time-travel. The key here is the reference to "one day" and the color purple, which presumably applied to the lightning the speedster generated - the only thing most people notice as a speedster moves. It seems clear then that his "weird purple speedster" is a reference to Iris West-Allen on the one day she accidentally acquired Barry Allen's super-speed, in the season 4 episode, "Run, Iris, Run."
Another point in favor of the Purple Speedster being Iris rather than Nora is the fact that Nora's lighting alternates between purple and yellow - the colors generated by both of her parents. It is also worth noting that, in the new timeline Nora established by going back in time to help her father, she's become a prominent hero in Central City in 2019 and been reported on repeatedly in the news. Barring another Flashpoint incident, there is no way there wouldn't be some historical record of XS in The Flash Museum for Nora to see in the modified timeline. It seems clear then that the mystery speedster is Iris West-Allen.