9. Kara Danvers and William Dey
Yes, I know that Kara Danvers and William Dey never officially became a couple on Supergirl, but a romantic relationship between the two was definitely the original plan - and the audience's response to that might have something to do with the reason that the show ultimately pivoted in a different direction.
When William first appeared on the show, he was arrogant, rude, and downright unlikeable, which made it hard to see why the show was teasing a romance between him and Kara. Not only was he all of those things to her, it was hard to buy that she would show any interest in him because of that. And that's not even taking into account how Crisis On Infinite Earths rewrote his backstory and somewhat altered his personality.
Fans did not like William and they did not like the idea of Kara dating him. He definitely grew as a character and was an enjoyable part of the show by the end of its run, but fans couldn't really get behind the potential relationship (even when the show continued to push the idea of them getting together in the future).
While Melissa Benoist teased a slow burn romance between them after the events of season 5, the subsequent announcement that season 6 was to be Supergirl's last killed the romantic subplot in its tracks, with the pair quickly agreeing to be friends instead. It then killed William in the penultiamte episode. Harsh.