343i’s Unyshek has confirmed that Halo Infinite will launch in 2021 and will be on XBox One (Original, S and X models) as well as Series-X and PC. So apart from the delay until sometime in 2021, there’s no other changes to their original plans at all:
To be clear about this: according to what 343i and XBox have told us numerous times before (in case anyone missed these or forgot these things):
(1) Halo Infinite is NOT a “next-gen game”, which is why it will NOT “look next-gen” on Series-X at launch. It is a current-gen game which will be “Optimised for Series-X” (hence a “cross-gen” game), as below.
(2) Only one of the four following features is required to get the “Optimised for Series X” title, but 343i have since announced they’re providing the FULL set of ALL FOUR “Optimised for Series-X” features (scroll down to see Infinite), which means the Series-X will have modes for:
(a) 4K/60fps Campaign (scroll down to “Now Entering Slipspace”).
(b) no Loading Screens or only very short ones thanks to the superfast next-gen SSD, as already seen in the Gameplay Demo (see “Now Entering Slipspace”).
(c) up to 120fps multiplayer.
(d) Ray-Tracing (originally planned for after Fall 2020 launch, as it requires a bit of an Engine Upgrade).
They’re also adding Smart Delivery (see “Now Entering Slipspace”), so you get the best version for your console (e.g. if you buy the game to play on an original XB1 or One S, then buy a Series-X later, you’ll get the right version of Infinite for each console, and for FREE - though obviously it’ll be a large download for the Series-X version!).
(3) Slipspace Engine was most likely designed to make the game on PC in up to 4K for the XBox One X (e.g. testing builds using a Scorpio XDK hardware Dev Kit), including scaling it DOWN to original XB1 and One S, but then also use a high-powered PC to scale it UP from One X to Series-X power once they knew what the final features and specs were and while they were waiting to actually get a Series-X to test builds on (e.g. the Gameplay Demo was done on PC). So, nothing is being “held back” by XB1, because Infinite isn’t using that instant “world-switching” tricks like The Medium, Psychonauts 2 or Ratchet and Clank which needs that superfast SSD or other such gameplay features that can’t work on an original XB1. Instead, the current gen consoles will almost certainly have the usual Loading Screens for Infinite, it is only Series-X that will have no Loading Screens, which is why there is no problem with older hard disk speeds (as wrongly claimed by some rumours and theories). PC games are made like this all the time to take account of a wide range of hardware, from budget to very high-end gaming PC’s.
(4) Some time after the originally planned Fall 2020 launch, Slipspace Engine and the game would get an upgrade to add a Ray-Tracing update for Series-X. This is standard on PC games at the moment - the game is made for everyone first, then after launch Ray-Tracing is added for the minority with Ray-Tracing hardware.
WHAT WE DON’T KNOW YET.
-
After launch, 343i plan to upgrade Slipspace Engine (e.g. eventually to Slipspace Version 2.0?). When it gets full next-gen feature support (full DirectX12 Ultimate and other new AMD Tech from FEMFX style Physics to Image Sharpening), will they “remaster” the original launch version of Halo Infinite to “look fully next-gen” on Series-X then?
-
When Slipspace gets upgraded for full next-gen Series-X features, will this mean new next-gen Campaign content launches by 2022 won’t support XBox One, One S and One X? Possibly, but that’s not confirmed yet. Even if it were confirmed, it may still be possible to stream that new content via XCloud.
I joke that 343i probably ran their plan past Blue Team concerning Halo Infinite running on the whole XBox family from XB1 to Series-X, and this was their reply: “I seriously LOVE this plan!”, so it got the Blue Team “Seal of Approval”. Who am I to argue with Blue Team? LOL! 
Hope this helps. 