https://www.gamespot.com/articles/halo-5-update-adds-xbox-one-x-support-and-makes-a-/1100-6454578/

Halo 5: Guardians might be more than two years old now, but Microsoft and developer 343 Industries are not finished yet supporting the Xbox One game. A big new update is coming out today, November 2, adding Xbox One X support, new game modes, and making significant and potentially controversial changes to weapon balance.

The update adds 4K UHD support on Xbox One X, though that won't matter much to the general population as the Xbox One X doesn't release until November 7. 343 says the game will look "better than ever" on Xbox One X thanks to the console's beefed-up horsepower. As for the new game modes, Oddball and Oddball FFA (with new medals, too) are now in the hopper. They were in the game earlier, but were later removed.