In terms of C++ version, it was proposed to target C++11.
GCC 5 has full C++14 support (one library functionality missing), and so does VS2015 onwards as well as Clang 3.4, see https://en.cppreference.com/w/cpp/compiler_support I doubt that any older compilers are in use _anywhere_ in reasonable numbers that this should constrain the development of CPython. While I don't know who proposed C++11 or where, I'd therefore like to propose to move to _at least_ C++14. Note that https://github.com/python/peps/pull/2309 already bumped the required C-standard to C11, and originally defined this as
The C11 subset are features supported by GCC 8.5, clang 8.0, and MSVC of Visual Studio 2017.
If those versions should be regarded as the lower bounds of compiler support (are they - or anything lower - tested on the build bots...?), then C++17 core language support would automatically fall out of this (there are some stragglers for full stdlib support, especially in clang; but that is usually not an issue). Best H.