everyone's talking about the new stage fright like vulnerability for iOS and recommending to update to iOS 9.3.3 but for those of us who are on iOS 10, are we safe? They don't mention anything about iOS 10
If 9.3.3 is patched to fix it then it's only safe to assume iOS 10 will be as well. Security fixes wouldn't just magically disappear out of a newer iOS version.
Developer beta 1 may not have had the fix included because 9.3.3 was still in beta then also. Now that 9.3.3 is live, the same fix has most likely been added into the latest iOS 10 beta builds.