It's surely not case that PS5 will just play any PS4 game, whether on disc or from the digital library
without any updates, but Xbox Series needs Microsoft to patch/update every game!?!
I get it for 360 and original Xbox games, the OS and hardware is in no way compatible, but Xbox One to Xbox Series?
Yes, you are correct that updates are not required to play Xbox One games on Series consoles.
So to add on to my earlier reply and to make it perfectly clear,
to play Xbox One games on Series X you do not need updates. I am checking the file info from a few of my older games to check installation date and update dates after I loaded them to make sure they still work. Updated shows "N/A" if the game has not been updated on that particular console. Install date shows the last time it was moved to that physical media, so if I move from 1 drive to another that will be updated.
Crimson Dragons: installed = 2018-07-05, updated = N/A, xdk, durango, Xbox one
Crackdown 3: installed = 2019-02-29, updated = N/A, xdk, durango, Xbox Scorpio
Forza Horizon 3: installed = 2018-07-05, updated = N/A, xdk, durango, Xbox one
Gears of War 4: installed = 2018-07-05, updated = N/A, xdk, durango, Xbox Scorpio
Halo 5 Guardians, installed = 2018-05-18, updated = N/A, xdk, durango, Xbox Scorpio
Now for a game I know have been updated to support newer DLC etc:
Minecraft Dungeons: installed = 2020-06-07, updated = 2020-12-17, xdk, durango, Xbox One
And a title which has updated to be Series X aware but not being GDK shows the following:
Forza Horizon 4: installed = 2020-11-10, updated = 2020-12-17, xdk Gen9 Aware, Scarlett, Xbox Anaconda
I really think a lot of this is very situational and they would be able to play Xbox One BC titles (physical and digital) that have SeriesX optimizations available online if they know how to manage the offline mode of the console. Since I have yet to see an organized and controlled environment test being done, I can't offer any additional comments other than people making too much out of sloppy and incomplete reporting and testing.