Without another name change, I don’t think that phrase will ever go away, for the simple fact that X as a name is too short and nondescript. In speech, X could refer to a someone you broke up with, or it could just be the beginning of another word, serving as a prefix. In text, it could refer to the actual letter itself, or the close button on a window, or a placeholder, or something NSFW.

There’s simply too many ways that X can be interpreted that even if people associate Twitter with X, people will still specify “formerly Twitter” just to avoid confusion