As noted on Reddit, first-generation AirPods Pro owners who also have the AirPods beta software will now see an "Adaptive Transparency" toggle in the AirPods section of the Settings app.
Apple debuted Adaptive Transparency with the AirPods Pro 2.The AirPods Pro 2 have an upgraded H2 chip that allows for Adaptive Transparency to work, so it is not yet clear how Adaptive Transparency with the H1 chip in the original AirPods Pro works in comparison.