Halo 5 developer 343 Industries announced a few months ago now that there was an Xbox One X update coming for the first-person shooter, which will provide 4K support and more on Xbox One X.
At the time, 343 Industries didn't tell us what other Xbox One X enhancements there would be, but now we have some more information thanks to a post on Neogaf by Halo Franchise Development Director, Frank O'Connor.
O'Connor posted to the Neogaf forums saying:
You'll definitely see significant benefits from running on HDR sets, and of course the game itself renders in 4K and takes advantage of higher resolution textures (will be a serious download) and assets, but HDR specifically was tricky for a couple of reasons specific to H5 - the first being of course that it's two years old and it's not just a switch - you have to "touch" all the content. The game was developed during the period HDR systems were still being ironed out. Even now there's a bunch of 10 bit/Dolby Vision etc issues for consumers.
This was largely resolved in time for Halo Wars 2, and so the HDR implementation for that will be full since the game was built from scratch to support it. The second - and from my perspective more important aspect is that Halo in particular has color symmetry on level designs as an aspect of balance and matchmaking. One side is blue and one side is red, often in fairly subtle ways. We already balance for that difference and applying changes retroactively would be a test issue at a grander scale than many other games. So we made the choice to concentrate on other IQ aspects for the update
Obviously that won't be an issue for future titles, since they'll be targeted for those systems in the first place and the art, test and design processes aligned.
And besides, it looks very pretty anyway.
There ARE methods for just blanket applying HDR to extant content, but the results are mixed and really dependent on what the underlying material looks like. So we developed for a conservative and safe outcome for that particular software.