Microsoft has confirmed that the Xbox Series S/X will play all Xbox One games at launch that do not require Kinect. Most already knew that Kinect games would not work as the Kinect system was not getting an update for the Series consoles.
This also appears to include Xbox 360 games as a Tweet Jason Ronald stated that all Xbox 360 and Xbox One games that don't require Kinect will work on the new Series S/X consoles. Although we do not have any details to confirm if this is truly ALL 360 games or just the majority that currently work on the Xbox One.
The Xbox team is proud of the fact that they have totaled over 500k hours into testing these older games and we are excited for a massive game library out of the box and a smooth transition onto the home theater shelf.
You can view a list of the backwards compatible games here, but we have noticed that this list does not appear include all 360 games (so we will have to see).