Runtime Versions and Platforms

There are different versions of the Objective-C runtime on different platforms.

Legacy and Modern Versions

There are two versions of the Objective-C runtime—“modern” and “legacy”. The modern version was introduced with Objective-C 2.0 and includes a number of new features. The programming interface for the legacy version of the runtime is described in Objective-C 1 Runtime Reference; the programming interface for the modern version of the runtime is described in Objective-C Runtime Reference.

The most notable new feature is that instance variables in the modern runtime are “non-fragile”: 

• In the legacy runtime, if you change the layout of instance variables in a class, you must recompile classes that inherit from it.

• In the modern runtime, if you change the layout of instance variables in a class, you do not have to recompile classes that inherit from it.

In addition, the modern runtime supports instance variable synthesis for declared properties (see Declared Properties in The Objective-C Programming Language).

https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/ObjCRuntimeGuide/Articles/ocrtVersionsPlatforms.html#//apple_ref/doc/uid/TP40008048-CH106-SW1