There doesn’t need to be such a flag because C++17 was fully backwards compatible with 14, with some tiny exceptions nobody cares about. Indeed, C++ has rarely

There doesn’t need to be such a flag because C++17 was fully backwards compatible with 14, with some tiny exceptions nobody cares about. Indeed, C++ has rarely made any breaking changes. A decade or so ago, GCC did cause some major ecosystem breakage, by cracking down on C++ constructs which had never been valid according to the spec but which GCC had previously allowed. When that happened, there was a flag to (at least partially) revert to the old behavior: -fpermissive.