Sasha Calle was cast as Supergirl in The Flash and there were big plans for her future in the DCEU. She signed on for multiple films and there were several conversations about her future in the franchise.
But, ultimately, after James Gunn and Peter Safran took over DC Studios to reboot the DCU, those future plans were flushed down the toilet.
In Gunn’s new DCU, Supergirl will be played by House of the Dragon star Milly Alcock. She will most likely make her debut in next year's Superman movie.