Thanks to the fact that Xbox Series X|S consoles support Microsoft's Universal Windows Platform (UWP), it's now possible to run the potent Xbox 360 emulator Xenia on modern Xbox systems – opening up a vast library of games that makes Microsoft's official backwards compatibility list look tame in comparison.
Xenia is already available on PC and Linux and is regarded as one of the leading Xbox 360 emulators. It is now available as a UWP app which can be installed on your Series X|S console – a timely arrival, given that we've recently had the news that a selection of Xbox 360 classics are being removed from digital sale, therefore putting them out of reach for modern players.
You can see the current state of Xenia UWP below. All the footage in this video was captured on an Xbox Series X:
Compatibility isn't perfect at the moment, but you can expect that to change as more development time goes into the app.
You can grab Xenia from its official project page or its Github page.