As much as that would annoy me if I owned one, it’s somewhat understandable I think given those games were designed before this card or its GPU family existed, and they can’t possibly test a huge back catalog from 5+ years ago
Important to note, this doesn’t affect online multiplayer or availability on newer consoles, eg you’ll continue to be able to buy, download and play 360 titles on Xbox One / Xbox Series.
MS probably wants to make sure that Series S doesn’t end up missing out on games or getting subpar experiences, given they promised their customers that it’s the same as an X just at a lower resolution. You can see how they want to avoid outcomes like Series S being confined to lower player counts, smaller maps, or other game-restricting features.
But they’ve painted themselves into a corner in this case. Split screen requires rendering the whole game twice, which the S isn’t powerful enough to do. It’s also probably a feature few players will actually use.
Seems like this case should be an exception to the parity requirements.