Halo: The Master Chief Collection is the most complete Halo package ever released, but it doesn’t have every one of Master Chief’s journeys included in it. Halo 5 is the only mainline game missing from the collection, and it’s going to stay that way.
Developer 343 Industries reaffirmed in a blog post that Halo 5 is not currently planned to be added to The Master Chief Collection. The wording suggests this is not a set-in-stone declaration, but for now, it seems quite unlikely that Halo 5 will join The Master Chief Collection.
As it stands, Halo 4 will be the final game added to the collection on PC. The Xbox One edition has included Halo 4 since release in 2014.