Hopefully, nobody else outside of the company is going to know about that interaction.

Anger is a form of communication. If anything, OP would be helping to send a message that if you push people far enough they will act out, maybe they shouldn’t be pushing other people quite that hard in the future. Boundaries are being crossed, and badly.

> it signals to others that you're not prepared to accept that kind of treatment.

It signals it in such a way that the employer will attribute your decision to your lack of professionalism and not to their own actions.

I think quitting is enough to signal you won't accept that kind of treatment. I don't think other people will treat you better as a result of adding a "fuck you" when you quit. I think it's more likely to make you look like the villain rather than the one who was being mistreated.

Quitting is "enough" to signal a myriad of things, but it just doesn't communicate any particular thing with any reliability. People quit for lots of personal reasons. "Fuck you" means they quit for outrage. Saying "I'm outraged" may presumably communicate the same thing, but as it does in notably tepid manner, it really does not.

Let's look at this situation again:

> I was leaving at a critical juncture and the company begged for me to stay at least for 3 months to complete an important project. I said "fuck you" and left.

Does the "fuck you" here signal unambiguously that this person is outraged at the way they've been treated? It could just as easily signal that this is an asshole who will screw you over when the opportunity arises.

Something along the lines of "I'm sorry but I can't continue to work under the conditions you've put me through the last 9 months" would be a much clearer signal. Maybe throwing in some swear words would improve the message, but a "fuck you" on its own leaves unnecessary ambiguity as to who the asshole is. Perhaps I'm overly concerned about my reputation, but I'd rather not add room for people to side with the person who mistreated me.

Well, I wouldn't have assumed that they said nothing more. I've personally never worked at a place where I only spoke my mind when handing in my resignation.