I think washing the mouth out with soap can be a good way to discipline children if it's done appropriately. Like, as calmly as possible and talking to them before, during, and after it, explaining why you're doing it. And don't overdo it. Like if using a bar, they should just have to hold it in their mouth for a few minutes. And it should be a new bar unless they said something really inappropriate on purpose or if it's a repeated offense; then a used bar is appropriate. If liquid, they should only have to swallow like a spoonful. Or you can brush their teeth with the soap in either case instead, so it gets stuck for a while. It's a punishment that kind of has to be done in the moment, so you might need to just use whatever's closest, maybe even detergent or another kind of cleaner; it can't be done completely safely, nor should it be. But give them water after only a few minutes too. Even show them the warning label on the soap if there is one and explain why they need to follow that at all other times but should ignore it for the punishment. The idea is to make them remember not to swear, but at the same time, not traumatize them; be reasonable about it.