Ubisoft has confirmed that it intends on sprucing up Rainbow Six Siege with PS4 Pro and Xbox One X support. Right now there's no word on when we can expect the update, although the publisher confirmed on Twitter that it's 'definitely on our road map.'
That's about all there is to go right now, although presumably we can expect the usual spit and polish typical of a Pro/X patch, namely HDR, 4K resolution, and other bits and bobs (or as I like to view it, anything that makes me feel like I was entirely justified in splashing out on the extra hardware muscle).
Right now though Ubisoft is understandably busy tinkering away on Year 3 of Rainbow Six Siege, and recently announced that it's launching three new versions of its squad-based shootathon. The developer also rolled out a new update for the game that includes a boatload of balance changes, so go check it out.
Rainbow Six Siege was released on PS4, PC, and Xbox One in December 2015 and has received steady support from Ubisoft over the past few years. From the look of things, there's no sign of that slowing down any time soon.