Back in content delivery for a second, yes I think ghosts is as close as you will get.
An example of how it will work:
* You buy Halo Infinite, and get a single disk/box/licence. You are getting however, two different games, that leverage two different platforms. One for Xbox one and xbox one x, and one for the new series x (and possible s).
* each version will likely be made by different teams with access to all the assets and code, and will essentially strip the game down to work on the older console. Good examples of this are COD Ghosts on One and 360, and Forza Horizon 2.
* the last gen game will be missing features. That much will have to happen. It will also have stuff cut back or cut down. It's just not possible else. You may have new loading areas, or less Onscreen ai, or anything.
* You will essentially have two games, but for the price of one - play the game on the system you own.
* while this is mandated atm for all first party titles, third parties are free to do what they want. For example COD may release and allow you to play the game on Xbox one and SX with a single disk, while battlefront may only work on SX with no last gen features. Meanwhile MOH may come out and work on both Xbox One and SX, but with two disks. Likewise, Farcry 6 may come out and be only on Xbox one (but would of course run better on sx). The choice is in the developers hands here.
* The general idea is you buy a single game, and pop it in. You don't need to do anything else. The system will instantly know the best version you want and install it, seamless.
* you may, however, be prompted to install features. For example, COD may ask if you want to install the single player, multiplayer, or both.
This is all really good and really smart. It may sound basic to you, but you can't do anything like this atm. You will get the chance on recent games to install only certain parts (for example Bayonetta/Vanquish allows you to install what you want), but the other stuff is new. The only thing that will make all of this even better is the option to download patches for games WITHOUT a disk licence. So, if you could download a day one 50gb patch for a game, before it's even released to buy on disk. That would help a lot. Even more so if you can download any game any time, fully, without the disk licence, but still be required to play. Though we know why they won't do that.