Most of C++ programs written before P0593R6 depended on implementation behaviour, and were graciously allowed to not be undefined behaviour just 5 years ago. C

Most of C++ programs written before P0593R6 depended on implementation behaviour, and were graciously allowed to not be undefined behaviour just 5 years ago. C++ as a language standard is mostly irrelevant, what one should care about is what the compiler authors consider valid code.