Maggie may have said yes to a future with Alex, but Floriana Lima didn't.
TV Line reports, Lima is not returning to Supergirl as a series regular so that she can pursue other opportunities. However, before she parts ways with the show, she will appear in more than one episode of Season 3 to wrap up the romantic relationship the National City police detective developed with DEO Agent Alex Danvers.
“We adore Floriana and have loved working with her to tell this inspiring story,” executive producer Andrew Kreisberg says in a statement. “Although she’s not available to us as a series regular next season, as she’s looking to pursue other opportunities, we’re happy she’ll be returning for multiple episodes in Season 3.”
In the Season 2 finale, after the Daxamites were defeated and fled Earth, Kara encouraged her sister to "never let [Maggie] go." So when Maggie goes to comfort her girlfriend, Alex blurts out a marriage proposal. Whether or not Maggie said yes or not is not revealed, but the unexpected proposal did put an ear-to-ear grin on her face.
Based on the aforementioned news, we can only speculate that Maggie either says no and goes back to being single, says yes and gets cold feet, or she doesn't live long enough to see the wedding reception.