Sometimes we'll get it wrong, and need to correct it. That's fine. We're capable of that.
What's not fine is throwing up our hands at even the concept of performing that judgment, because it's subjective or whatever. That cedes the public square to extremists, which results in a shittier society.
When we started this game, we had slaves, who were defined as three fifths of a human being for purposes of political representation of slave owners. Voting was restricted to white land-owning men. It's getting better.
"Be civil. Don't say things you wouldn't say face-to-face. Don't be snarky. Comments should get more civil and substantive, not less, as a topic gets more divisive."
"Please respond to the strongest plausible interpretation of what someone says, not a weaker one that's easier to criticize. Assume good faith."
Consider this a warning, as we eventually ban accounts of repeat offenders.
You're WAY too hung up on this "who decides?" question, ignoring the reality that either someone has to decide, or that no one should decide and society should simply not regulate speech in any way, ever.
Good luck with the latter if you ever have kids.