Its not like Microsoft gave them much leg room after the fan backlash to halo 4.
That was a long time ago and it was still just a rumor. It was somewhat implied by lore released in a waypoint canon fodder that at least the Grunts had joined Cortana, meaning its not a far stretching to imagine that other races also would.
(i currently cannot find any source so do not quote me on this)
Finding a way to force the flood back into a already full story wouldnt make sense. Unless it has a longer campaign or resolves the Created issue within the first few levels
They found a nice balance with halo wars 2 but some people like me like the 343 art style fine. And i would be rather annoyed if they went back to something like reach's. Going forward with
From what i can tell there is no evidence that the banished will show up for halo 6. I would enjoy to see them but i would rather not see them pointlessly jammed in with some stupid reason with spme dumb explanation as to how and why the left the ark. I made my thoughts on the flood clear earlier in the post.