In watchOS 8 and below there’s a neat feature for Workout called Power Saving Mode. When enabled, the heart rate sensor (and cellular, if you have it) is disabled during walking/running workouts to conserve battery. This is extremely helpful for long walks or hikes (5+ hours), where continuous heart rate monitoring is a major source of battery drain.
In watchOS 9, Apple removed Power Saving Mode and replaced it with Low Power Mode. Low Power Mode can be enabled independently of Workout and disables most background tasks which makes it much more consistent with iOS. However, unlike Power Saving Mode, it doesn’t disable continuous heart rate monitoring during workouts.
To bring back the functionality of Power Saving Mode during workouts, you also need to enable Fewer GPS and Heart Rate Readings (under Settings -> Workout). This does what it says on the tin, and has advantages over Power Saving Mode as you still get some heart rate measurements instead of none.
While Low Power Mode is available on all watches running watchOS 9, Fewer GPS and Heart Rate Readings is only available on current generation watches (SE gen 2, Series 8, and Ultra). Older watches don’t get this feature. So after upgrading a Series 7 or below to watchOS 9, you lose the ability to kill the biggest source of battery drain during long workouts*. Of course, once you’ve figured this out, it’s too late. In typical Apple fashion you can’t downgrade watchOS; you can only avoid upgrading.
Apple removing features or only giving new features to new devices isn’t exactly news. However, I find this case particular egregious because the feature wasn’t “removed”; it was replaced with a slightly modified version which is now only available on current generation watches. Also, this feature just collects data from a couple of sensors on an interval (opposed to continuously). The watch already does this at rest, why can’t it do the same during a workout? This isn’t a hardware limitation. Removing a power saving feature from old devices – the ones that need it the most – is a quintessential example of planned obsolescence in action.
* There is somewhat of a way to restore the old functionality: by manually disabling the heart rate sensor under Privacy settings before long workouts. This is pretty hacky, but the fact that Apple wants me to buy a new watch to restore functionality I previously had on my current watch had gives me all the motivation I need to keep my Series 4 going as long as possible. Even if it means digging through settings to manually disable certain sensors before long hikes.