While the Halo Master Chief Collection will be getting a massive next-gen upgrade, Halo 5 will not.
343 confirms no upgrade
In a recent blog post developers 343 noted that Halo 5: Guardians will not be getting a full optimization for the Xbox Series X & S.
The MCC's upgrade list includes faster load times, cross-generation play, adjustable FOV, an improved splitscreen experience, and up to 4K resolution at 120FPS on the Xbox Series X or 120FPS at 1080p on the Xbox Series S.
However, Halo 5 will still benefit from the new consoles with faster load times and consistently higher resolution gameplay.
Why no upgrade?
343 didn't say why Halo 5 wasn't getting the same treatment as MCC, but we can theorize.
Of course, the big focus is on Halo Infinite. Delayed until 2021, the title is expected to be an enormous hit for Xbox thanks to the new Slipspace Engine and taking full advantage of the raw power the Series X has to offer.
It could be that 343 simply couldn't spare the resource to upgrade Halo 5 too.
While there are no current plans for Halo 5 to join the MCC, fans expect it to happen at some point. That could be when the game gets its major upgrade.