Crisis on Infinite Earths left the heroes of the Arrowverse with a lot of fallout to deal with — but Supergirl season 6 just retconned and fixed one of the biggest lingering issues. Kicking off in December 2019, Marv Wolfman and George Pérez's seminal storyline was adapted across five episodes. Folding in various corners of a wider DC universe, the saga saw a group of heroes known as Paragons (and their friends) battle the Anti-Monitor (LaMonica Garrett). Despite failing to protect the multiverse, they successfully reconstituted it (unbeknownst to them) and merged their own respective worlds into the newly-crowned Earth-Prime.
Ever since then, each show has dealt with a number of consequences regarding the new status quo. Once defeated villains have reemerged with new faces and/or more powerful than ever. Doppelgängers from fallen Earths arrived to either complicate matters or seek vengeance. Established dynamics shifted, seemingly turning friends into foes and vice versa. And the heroes have had to navigate a world in which even its general geography had become foreign to them. That was especially true of the aforementioned Paragons, who only retained their Pre-Crisis memories. Ultimately that complication extended to the respective member of each show's team. As Martian Manhunter (David Harewood) ventured around and restored everybody to their former selves, so did their memories of the new timeline seem to be erased.
That was most prominently established via The Flash season 6, episode 10, "Marathon". Speaking with Frost (Danielle Panabaker), Cisco Ramon (Carlos Valdes) lamented the new world order. He'd had to not only strenuously research and catalog all the new (or altered) villains, but map out all the new locations of Earth-Prime. Such endeavors would later take him on a trek around the world in order to be comprehensive — which, given Cisco's level of genius, he wouldn't have had to do if Earth-Prime memories were intact. As if that wasn't proof enough, he further demonstrated his unfamiliarity with the new timeline by revealing that he was wearing a Superman T-shirt that he had no recollection of ever purchasing. Things across the Arrowverse remained largely consistent with that sense of obliviousness. In Supergirl season 6, episode 5, "Prom Night", however, the notion was officially retconned.
The episode saw Team Supergirl research the best point in time to visit to collect a sample of her DNA. As they did so, Alex Danvers (Chyler Leigh) illuminated them as to Young Kara's (Izabela Vidovic) teen exploits with Kenny Li (Peter Sudarso). At that point, J'onn interjected, recalling that Kenny was supposed to have died in High School. "No," Alex replied. "That's one of those old-world, new-world differences. In this world, Kenny never got killed." Of course, it could be argued that Alex merely did similar homework as Cisco. Given how vividly she recalled the details, however, and the fact that the parts of Kara's life she recounted wouldn't have been documented, that'd be unlikely. As such, not only did Supergirl season 6 retcon Kara's (Melissa Benoist) past but an entire element of the Arrowverse's recent mythology.
As it happened, though, continuity error aside, it's a move that was likely for the best. Having the Arrowverse heroes faced with an unfamiliar world no doubt provided much material to mine both comedically and dramatically. In the long-term, however, it was a needless complication — especially since, as with the Supergirl episode, it was only a matter of time before a character's past was visited or otherwise came into play. Equally, the overwriting of entire memories carried the connotation that the Arrowverse characters had stolen alternate version's lives. And the longer that went unaddressed, the more egregious it seemed. While sidestepping the issue wasn't a perfect idea, it was more palatable than nothing. As such, while it stood as a firm retcon, Supergirl season 6 establishing that both sets of memories exist simultaneously served as a solid solution for the Arrowverse to go forward with.