In early 2024, Apple removed a blood oxygen monitoring feature from its then-current Apple Watch Series 9 and Ultra 2 watches, following a ruling that the feature violated light-based pulse oximetry patents from a California-based company called Masimo. Removing the feature allowed Apple to circumvent an import ban and continue selling the Series 9 and Ultra 2 (and, later, the Series 10) in the US.
Today, Apple announced that it's restoring a version of the blood oxygen monitoring feature to affected Series 9, Series 10, and Ultra 2 watches. Apple says these updates were "enabled by a recent U.S. Customs ruling."
The feature has been redesigned, presumably to get around the Masimo patents that prompted the import ban in the first place. Apple says that sensor data from your watch will be sent to your iPhone for processing, and you can find the data in the Respiratory section of the Health app on your phone. The old version of the feature allowed results to be viewed directly on the Apple Watch, as shown in Apple's documentation.