HTC has issued an update to the developer and unlocked editions of the HTC One M8. The update, software version 1.54.1540.9, comes in at 117.98 MB and contains the Extreme Power Saving mode and camera and gallery feature enhancements.
The Extreme Power Saving feature was not available in the version of the One that shipped to carriers in the United States at launch, though One M8 users outside the U.S. were able to use it. The mode appeared on Sprint's One early last month, and the Candian model last week. The Extreme Power Saving mode allows the One M8 to shut down everything but the essential functions of the phone in order to preserve battery life.
Thanks to luckydwg in the Android Central forums for the tip!