This is cool for former Smalltalkers: "Advanced class redefinition" (there was previous attempt at http://dcevm.github.io/ to bring this to HotSpot, but Sun/Oracle decided not to increase HotSpot complexity - see https://openjdk.java.net/jeps/159). So it seems JVM is one step closer to the Smalltalk VM...
This kind of stuff used to work in Objective-C too, but it was removed step by step for performance reasons. If you allow it, for every call you do to such an object you have to first check if the method exits on it or somewhere up the inheritance chain. You can cache of course, but adding or removing a method to an object invalidates the cache for the object itself and everything that inherits from it's class.
In a modern JIT with dynamic decompilation you can do these things in place, because the compiler essentially has global knowledge. The only real issue is to define the semantics (and then implement it of course). Objective-C is AOT compiled so the runtime checks are needed all the time.
I'm not sure why you think it's been removed? The entire runtime is still there…the cache you're talking about still exists and is hit for every method call.
