Generations ago spanking enjoyed almost universal cultural acceptance but, in more recent decades, cultural attitudes have shifted pretty hard against it and the prevalence of parents who are fully open and upfront about spanking their kids has declined pretty steadily. Nowadays I assume the vast majority of parents who spank are far more discreet about it in comparison to previous generations. Given the controversy surrounding this practice, I suspect the actual prevalence of spanking is greater than what you might assume given current cultural attitudes. It is entirely possible you may know some spanking parents and not realize that they discipline their kids in this way.
How common do you think spanking actually is now despite its' significantly diminished social acceptability?