If I were to pick one of the classic Star Wars characters that was underserved by the sequel trilogy it would be R2-D2 without a doubt. George Lucas has said that in many ways his Star Wars saga was almost told from the point of view of the droids because R2-D2 and C-3P0 were involved in most of the adventures. However, with this new trilogy of movies new droids have been introduced and now it seems like the two classic droids get a back seat.
Now in the The Rise of Skywalker it could be possible that we see even less of R2, because now we have another new droid called D-O alongside BB-8 and there has been almost no marketing material showing R2-D2 at all.
R2-D2 is limited to just seconds of screen time in both The Force Awakens and The Last Jedi. He is given two fairly important moments, but other than that he has very little to do. My hope for The Rise of Skywalker is that the reason R2 has been sheltered from marketing so much is because his involvement in the movie is a spoiler, which would mean he’s got an important part in the movie.
R2-D2 has been the macguffin (an object that is nessesary to move the plot and characters of a movie) in a movie in the past, in the first movie A New Hope. R2 is known to hold lots of important information about the Empire because of his adventures in the sequel trilogy. So, with the Death Star II likely to be an important part of this movie it could be that R2 has more information about it in his storage and both the First Order and Resistance are trying to track it down.
Or it could simply be that R2 isn’t in the marketing very much because he just simply is hardly in the movie at all. Which would be a bit disappointing for original trilogy fans.