Star Wars is a fantasy, but using this trope is not what makes it so.
Star Trek uses the precursor race thing a little bit, and Mass Effect goes all in on it. It’s definitely more of a trope in fantasy than scifi—scifi is more likely to use the related-but-distinct trope of a pre-existing but still-ongoing technologically superior empire (Mass Effect also does this)—but neither trope is exclusive to the genre.
Looking at how often this happens in Star Wars proves that it’s space fantasy.
It’s a space opera!
Everything about Star Wars proves its a space fantasy. I mean, its got wizards in it!
Star Wars is a fantasy, but using this trope is not what makes it so.
Star Trek uses the precursor race thing a little bit, and Mass Effect goes all in on it. It’s definitely more of a trope in fantasy than scifi—scifi is more likely to use the related-but-distinct trope of a pre-existing but still-ongoing technologically superior empire (Mass Effect also does this)—but neither trope is exclusive to the genre.