Yes I can add new features to some files or I can compile with c++17 and link to an old C library. What I can't do is remove features or change the default cal

Yes I can add new features to some files or I can compile with c++17 and link to an old C library. What I can't do is remove features or change the default calling conventions/ABI and still link with old code. If you declare in the interface and in the code that this is new code with new rules then this is possible. When you cross library boundaries the compiler adjusts. That is what rust allows and what C++ could do.