Complex sentence: At WWDC 2016, Federighi introduced iOS 10 and macOS 10.12 "Sierra" and said that the 15-year-old OS X would be rebranded as "macOS" in tune with the naming scheme used for iOS, tvOS, and watchOS.
Split into simpler sentences: At WWDC 2016, Federighi introduced iOS 10 and macOS 10.12 "Sierra". He emphasized that after using the Mac OS X and OS X name for 15 years, OS X would be rebranded as "macOS" in tune with the naming scheme used for iOS, tvOS, and watchOS.