2019's Star Wars ever since the original movie in 1977, and is one of only two characters to appear in all nine episodic films in the franchise (alongside his old friend C-3PO). Artoo has played a crucial role in the fight to restore peace and justice in the galaxy, helping to bring down the Empire and saving the lives of several major characters, while also providing the blueprint for how to be an entertaining, humorous droid character at the same time.
Disney's sequel trilogy learned the last part of that pretty well, bringing in BB-8 to effectively be The Last Jedi, including an emotional reunion with Luke where he replayed Leia's message requesting hope. The Rise of Skywalker, however, couldn't offer as much.
The droid does get to be in a couple of key moments in the final Skywalker saga movie. R2-D2 is present when Leia dies, adding even greater tragedy to his character arc and mirroring the fact that he was also there when she was born. He also gets a brief moment of glory with C-3PO, helping to restore his memories after he's undergone his latest wipe. But that's about it and, save for flying in Poe Dameron's X-Wing during The Rise of Skywalker's final battle for some reason, he doesn't get a particularly significant role to play. That's even worse by the end, when he just briefly partakes in the celebrations, but his journey is given no sense of finality; his arc does not feel complete, and there's no real send-off for him.
For all of The Rise of Skywalker's faults, it does at least give closure to several stories, and for its new characters balances that with a sense of where things could go in the future - most specifically with Rey, but it's easy to imagine more adventures for Poe and Finn in Star Wars movies, while even Chewbacca gets a medal as an attempt at an ending. For R2-D2 (and, ittedly, C-3PO, though he at least gets more to do overall in the movie) there is no such luck - he's just there, and audiences are left to assume he just exists around the Resistance or whatever new New Republic forms in the wake of the First Order's defeat, perhaps gathering dust once again. For a character who is as crucial to the Skywalker saga as almost anyone not called Luke or Anakin, that isn't really good enough, and he deserved some touch in his ending that offered up a much more fitting sense of finality.
The best way to do this would've been for The Rise of Skywalker to use George Lucas' original ending twist, which would've seen R2-D2 in the future telling the story of the Whills. It's a bold move, but the narrative device would actually fit with how the saga is presented and Artoo's role in it, and given him a satisfying close that brings everything full circle. Even without going that far, though, The Rise of Skywalker should've been able to provide a greater, more specific moment for him at the end to really bring his journey home, given everything he's been through and fought for. Unfortunately, though, it seems that Disney forgot about R2-D2 even in the final moments.
