While I agree with you in principle, remember that some users are literally children. Should they have a toggle for “I am an adult”? Yes. But it does make sense to have some accommodation for users who don’t find profanity appropriate yet.
Sure, but one could easily say plenty of threatening, illegal, manipulative things to a child without throwing an f-bomb. In fact getting a child to let down their guard probably works better that way. My only point is that curse words hardly ever hurt a kid, and you don't need curse words to hurt a kid the worst.
It may be harmless in some regards but in some social spheres it will be punished (e.g. kids bring punished for swearing at school) and in others the consequence of swearing will be silent exclusion (e.g. not being invited to meetings or asked to lead efforts).
Those seem like harms to many regardless of feelings about language restriction.
I swear like a your-favorite-stereotype. When my kid was maybe 4, I told her "I don't care if you talk like I do, but people will give you trouble for it". Probably with more detail than that; it's been years now.
She fully understood the point, right then. She also had no problem with other advice about how people would react to whatever.
She's 17 now. I actually don't think I've ever heard her utter a "swear word".
Kids, in general, have no problem with the idea of social context.