To my knowledge there were only a couple of things 'removed' from C, but every time the question comes up I forget what they are. They were removed precisely because the committee observed they were very seldom used features, and wouldn't break much of existing code, where any modification required was minimal and local. It was such a trivial thing that I dare say most developers, I among them, can't manage to recall just what it is.

There may be details in implementation whereby the claim "any" C program could be denied, and we'd have to say merely "most" C programs could be compiled in C++ and produce a functional executable (meaningful, a curious choice, I think means a running executable).

I'd go so far as to say you'd have to search far and wide to find a C program that wouldn't compile, but there may be some linker complications. Objective-C doesn't use the same kind of name mangling that C++ uses, so perhaps there could be situations where shared libraries, static libraries or DLL pre-requisites would not always work, but the alterations would be minimal.

Objective-C developers generally hold the opinion of the superiority of that language, especially of the message paradigm and dynamic typing. Most of the reported advantages are not clear cut, and several of the reported exclusive features of Objective-C are only exclusive if the comparison insists on base language constructs, ignoring libraries and modern generic programming methods, and even when the results are comparable there can be continued denial for trivial reasons.