https://wccftech.com/xbox-series-x-s-fps-boost-older-xbox-tech/

The Xbox Series X | S FPS Boost feature will focus on Xbox One games, but there is still the chance that it might be used to boost the performance of original Xbox and Xbox 360 titles.

Speaking during the latest Kinda Funny Xcast, as reported by Pure Xbox, Xbox's Jason Ronald confirmed that older Xbox titles are still under evaluation. Right now, they are focusing on Xbox One titles as the technology works best with the previous generation games.