It's also pretty easy to avoid. I don't think any parent would do it without lots of warning.
It's a parent's obligation to find consequences for their child that aren't demeaning.
And after soap in the mouth what's next?? It's not demeaning at all. I don't know how you even get that conclusion. Children who use proper language and observe the boundaries set forth for them will never even have this happen. Decisions have consequences. If a child gets soap once or a belt a few times, its not the end of the world and the child will be better for it, if the parent handles it properly.
I find your view sickening.
What I find sickening is seeing children, teenagers, and young adults who have no concept of discipline, decorum, or common decency.
I had my mouth washed out with soap once by the most loving, caring, wonderful mother a child could ask for. I certainly deserved it- but all I kept thinking was, "this doesn't make any sense"- meaning, this doesn't reflect my mother's character. She felt uncomfortable about it, and I felt confused and alone.
All that aside- any good parent, who has raised their children correctly and without harsh punishments, will not have children with the characteristics you describe. The only circumstances under which I would wash my kid's mouth out with soap is if he started to turn into a boring, mindless, automaton, more concerned with society's standards of "decency" than with cultivating his genuine self. I hope to have punk rock children with a sense of morality that goes beyond "decorum".