The original idea mandated by Microsoft was that Xbox games need to support Series S as baseline, and then the Series X version would just be prettier/faster than Series S. But the two versions are expected to have the same features/compatibility.
Dropping splitscreen support for just the Series S but keeping it on the Series X seems like it might run afoul of Microsoft's policy in that regard, but I think Microsoft is likely to let it slide given the embarrassment of not having what is likely to be the most popular game of the year on their console at all otherwise.
I didn’t read it that way. Of course Microsoft made the call to let it happen, but they may have stuck to their guns if this weren’t the biggest game of the year.
The original idea mandated by Microsoft was that Xbox games need to support Series S as baseline, and then the Series X version would just be prettier/faster than Series S.
If they wanted that they needed to offer CPU parity.