I'll turn this around: you've never told someone to go fuck themselves?
I would say that I've used it more IRL than online!!
I don't think it is a thing to commonly use in conversations, no. When people are discussing things in good faith, it is inappropriate. But there are times when it is appropriate, IMO. You're having a good faith discussion here I think, if I just replied "fuck you" yeah. It would be pretty mean.
Also, sorry, I should have elaborated: by "fuck you" I don't always mean literally fuck you. I also mean phrases like "You obviously don't want to have a conversation about this" etc.. which are dismissive and against the guidelines of HN (I have been rate-limited for this type of speech, btw)
Sometimes it is appropriate, even if that appropriateness is only like you're saying: advertising to people like you that you don't want to converse with me. Good! :)
Much worse for me to just "obey the rules" and troll you in good faith while adhering to the guidelines, IMO. Much more toxic, too.
It is like your coworker who, every time you try and have a constructive conversation with, says "I'll set up a meeting to talk about this" or "we'll discuss it on standup" or "send me an email about this" or whatever - and never follows up.
Polite, appropriate, work-safe - but much more toxic than if they would just say "we're not doing that b/c of XYZ, sorry" (which is long-form for "fuck you").
So yeah, it should be OK to say "fuck you" on a healthy forum! ;)