As of previous first beta launches after a major release and new hardware features, this beta has disabled blood oxygen sensor by removing certain functions from both the watchOS (documented on release notes) and from iOS (not documented on release notes). If you have not yet deployed watchOS 7.1 beta on your watch, you can restore iOS 14 on your phone to regain access to this feature; otherwise, welcome to the club, and take a seat while we wait for the next watchOS 7.1 beta and iOS 14.2 beta that will restores the blood oxygen sensor capabilities.