True, but none of the 14 listed principles is absolute. C2x compilers probably would (and could be encouraged to) accept old-style declarations and definitions

True, but none of the 14 listed principles is absolute. C2x compilers probably would (and could be encouraged to) accept old-style declarations and definitions in some non-conforming mode. They could even be accepted in conforming mode as long as the compiler warns about it (only a diagnostic is required).