By "completely trivial" do you mean that the constructors are unnecessary? If so, then yes: they won't be used in most solutions.

The kata is reported on the list of kata to fix, but for different reasons (although slightly related).

Sure. I'll raise an issue about this now (it's suprising nobody did it before).

join and split strings is a trivial task for a 2kyu kata, moreover this is how the kata is designed, one must handle the input the way they want (there is no obligation in using this to solve the kata). I cannot see the using namespace std;, ok it's a bad practice and it probably should be removed but this is completely independent from providing helper functions.

Why should it be?