I’d argue it doesn’t make sense to fix c++. A language is not about what you can do with it, its about what it doesn't let you do. C++ will continue letting you do everything in the name of backward compatibility.
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.
Actually, that point has already come: There is insistence on maintaining ABI compatibility with increasing versions (as opposed to source-level compatibility), which prevents various potential improvements.
Regardless - C++ will effectively has some deprecation, at least of the standard library, with the introduction of modules instead of included headers in C++20.
