watchOS 9.1
Today’s update introduces an extended battery life option when using Outdoor Walking, Running, and Hiking Workouts. You can opt to reduce the frequency of heart rate and GPS readings if you have an Apple Watch Ultra, a second-generation Apple Watch SE, or an Apple Watch Series 8.
I wonder why this is limited to the new models. The hardware is not that different, and choosing how often to read a sensor seems more like a software feature than a hardware one, anyway.
This document describes the security content of watchOS 9.1.
Previously:
I call it a feature limitation to force coercive marketing.