https://www.gamespot.com/articles/microsoft-taking-extra-time-for-halo-backwards-com/1100-6453207/

Earlier this year, Microsoft announced that four Halo games--Halo: CE Anniversary, Halo 3, Halo 3: ODST, and Halo 4--are coming to Xbox One through backwards compatibility. That still hasn't happened, and developer 343 Industries has now given another update on the matter--and it sounds like the games should be added to the catalog soon.

In its latest weekly blog post, 343 said it understands that players are "very eager" to bring the four Xbox 360 games to Xbox One. The backwards compatibility team is "hard at work on final polish," 343 added, so it sounds like the release is coming up shortly.