IIUC Because C++ has pointer provenance but no explicit mechanism for tagging or clear instructions about what is guaranteed, this is basically always all Undefined Behaviour and yet in practice lots of this will work on real C++ compilers.

Today in C++ the way to avoid technical UB for such tricks will be going via an integer which is promised to have the same width as a pointer. In practice the compiler won't do anything different (types evaporate during compilation) but that isn't UB.