https://wccftech.com/xbox-series-x-backward-compatibility-2016/

Backward Compatibility has been one of the main Xbox focuses in the past few years, and it will remain to be so for the Xbox Series X and Xbox Series S generation. So much, that the team has been working on it for the next-generation consoles since 2016.

Speaking with Inverse, Xbox Director of Program Management Jason Ronald confirmed that the team has been working on the Xbox Series X and Series S backward compatibility since 2016, running performance captures of gaming on a simulator of the next-gen chip.