They're called "editions". And it was proposed to the C++ committee, but with things like ADL and SFINAE existing, making any substantial changes just isn't possible. What happens if you SFINAE on a class derived from an STL container with a method removed in Y from a module in edition Y when it is declared in a module with edition X (where it would exist)? Does the method exist or not? When using contracts from different editions which holds? In what context is the question asked?

Even things like integer promotion rules or "trivial" things like "deprecate NULL and 0 for nullptr" are likely untouchable with any edition mechanism for C++ that doesn't end up Breaking the World.