>>212485939Supergirl shows up before the end, stepping out of a yellow taxi driven by an alien with green skin, one eye, and tentacles. She’s clearly drunk and has come to pick up Krypto, thanking Superman for taking care of him all week while she was partying in the Nebula Galaxy. The red sun there allows her to get drunk—something she can't do under Earth's yellow sun. She's a party girl at heart.
She's heading off to celebrate her 21st birthday on another planet and invites Superman to come along, but he says he's busy. After she leaves, he tells the computer to show him his parents. This time, it displays the Kents. Earlier in the movie, it had shown his Kryptonian parents—who, in this universe, are conquerors. He had rejected that part of his heritage.
That’s why the movie is called Legacy—it’s about his human side.
So yeah, her appearence is direct setup for the 2026 Supergirl movie.