Who defines "polite"? Who defines "agitated"?

One person's "asshole" is another person's "passionate".

I imagine the moderation team would? What else did you have in mind?

Hacker News actually already has that to a first approximation (though if I understand correctly, rather than being applied to heated or controversial threads, it's applied to users who have difficulty consistently adhering to the site guidelines. Their accounts are flagged as not allowing more than a certain number of posts per hour).

Much better: bots insert fake responses that are calming and neutral. If you want to tailor/manage discussions (manipulate might be a better word), it seems like adding to the discussion is far better than censorship (or rate limiting as you call it). People being shadowbaned or rate limited or whatever just aggravates the problem because it makes people more hostile in general. Don’t censor bad speech, promote what you believe to be better speech.