Objective-C headers for Apple's iOS, macOS, tvOS and watchOS frameworks. I will update them from time to time, as new versions of Xcode are released.
iOS 2-12: https://github.com/nst/iOS-Runtime-Headers
iOS 6: https://github.com/steipete/iOS6-Runtime-Headers
iOS 8: https://github.com/yangxt/iOS8-Runtime-Headers
iOS 9: https://github.com/EthanGHub/IOS9.3-Runtime-Headers https://github.com/ksenks/iOS9-Runtime-Headers
iOS 10: https://github.com/JaviSoto/iOS10-Runtime-Headers https://github.com/shaojiankui/iOS10-Runtime-Headers
iOS 11: https://github.com/iuzumakin/iOS11-Runtime-Headers
iOS 12: https://github.com/eziochiu/iOS12.1.2-RuntimeHeaders
iOS 13: https://github.com/eziochiu/iOS13.4.1-Runtime-Headers
iOS 14: https://github.com/SparkDev97/iOS14-Runtime-Headers https://github.com/conorthedev/iOS14.2RuntimeHeaders