Zack Snyder's Justice League has a callback to that mysterious Supergirl tease from Man of Steel. Throughout Snyder's DC movies leading up to his Justice League adaptation, the filmmaker hasn't been shy from teasing and setting up Easter eggs for other big characters in the comics. While Kara Zor-El never showed up in Man of Steel, the director had helped pen a prequel comic that established her place in the DCEU. Ahead of the film's release, Snyder and David S. Goyer co-wrote a prequel story that was released as a limited comic that chronicled the events that led to the Kryptonian scout ship arriving on Earth.
In the DCEU, the scout ship became Superman's own version of the Fortress of Solitude as opposed to the traditional crystal construct that is seen in other mediums. In the tie-in comic, the plot follows Kara on her mission with the ship's crew to go explore other worlds that Krypton could inhabit. But a villainous Kryptonian named Dev-Em killed all the other crew members that had gone into cryostasis except for Kara, who wakes up much later. When Kara awakens, she learns that her crew has been murdered and that she's on her own against Dev-Em. The two eventually have a showdown while the ship crashes on Earth in Ellesmere Island, Canada.
The Man of Steel prequel never clarifies what happened to Kara or Dev, except showing a shadowy figure exit the ship and wander away. The ship is later found by Clark thousands of years later, and while he goes past multiple pods, Kara's opened one is spotted. While it has been almost a decade since the film and prequel comic came out, the mystery pod has yet to be addressed in the later films. Snyder has always been on the record saying that it was meant to be a big DCEU setup without confirming or debunking the idea that this was their build-up to Supergirl. However, in Zack Snyder's Justice League, it gets seen once again, thus implying the Supergirl reference wasn't a one-time Easter egg.
When the Justice League agrees to resurrect Superman after his passing in Batman v Superman: Dawn of Justice, they use a Change Machine, the final Mother Box, to bring him back. But to do that, they needed access to Kal-El's ship, and when they enter it, Kara's pod is the first thing that is spotted. All of the heroes walk past it as well as the other pods with some of the crew members' skeletons still inside. It doesn't feel like a convenience that Snyder highlighted Kara's pod because obviously it was meant for a bigger payoff in his DC vision.
It's a tricky situation whether or not that Easter egg will ever get its payoff since the Snyder Cut isn't part of the DCEU canon. The Flash movie will be introducing Supergirl with Sasha Calle playing the Kryptonian heroine. There's always the potential that director Andy Muschietti will honor that setup by revealing Kara's fate in his film. Since Warner Bros. isn't planning on continuing Snyder's DC universe, it's hard to say if his tease of the pod that was meant for a bigger DCEU setup will ever get to come to fruition after Zack Snyder's Justice League.
