Aside from Supreme Leader Snoke offhandedly mentioning the Knights of Ren during Star Wars: The Last Jedi, there weren't any direct references to them in the movie, so one couldn't be faulted for possibly forgetting that they're floating around somewhere. As Johnson noted, the Knights could have replaced the Paretorian guards, but considering how important the Knights are suggested to be, as well as how Rey and Kylo ultimately killed all the Praetorian guards in Snoke's throne room, using the Knights instead would have been a waste of their potential. However, before delivering the above quote during his interview with The Empire Film Podcast, Rian Johnson said that he does find the Knights of Ren interesting "intriguing who they are." So if director J.J. Abrams and writer Chris Terrio choose to include the Knights in Episode IX, Johnson is curious to know more about them.