Different states have dramatically different taxes. Some states have no income taxes.
So in one sense, it's a "solved problem", with the solution being that licensing requirements would be dramatically different based on locality, but in the sense that people want — a uniform standard for hiring software developers — it's not solved at all.
> but in the sense that people want — a uniform standard for hiring software developers — it's not solved at all.
Exact same thing can be said about the hiring process in general. Taxes and employment legal contracts vary from state to state. I am sure employers would love a single contract, no matter where they hire an individual, but that is not currently the case.
So in one sense, it's a "solved problem", with the solution being that licensing requirements would be dramatically different based on locality, but in the sense that people want — a uniform standard for hiring software developers — it's not solved at all.