Re: Who decides the definition of "Harmful"?
Those other ideas collapse when revealed as untruths.
I am firmly in favor of strong protections for freedom of expression, and automatically hostile toward any censorship system. But this comment is simply incorrect, as a vast number of methodologically-sound psychological experiments and the vast sweep of human history both attest.
Even testable false hypotheses don't show any sign of being overwhelmed by truth. Take, oh, the Flat Earthers. Or the homeopaths. And of course untestable hypotheses (religion, conspiracy theories, solipsism, etc) cannot logically be refuted.
Education helps. Some economic pressures can help - though it's difficult to institute most of those without unacceptable constraints on expression. For the most part, though, we have to bear the costs of significant numbers of people believing false ideas and acting accordingly, as the price of freedom of expression. That's a trade-off inherent in the human condition.
It's a wind-eye. A society can choose to be blind but sheltered from the cold, or confront the gale and see.