Once you draw an arbitrary line, it’s not free speech anymore.
The only line you can draw and still call it free speech is to forbid speech that causes credible, immediate harm to someone, like yelling “fire” in a crowded theater or engaging in slander. Everything else is fair game, no matter how distasteful it may be.
Berkeley’s idea of prohibiting what they consider harmful speech (meaning speech that doesn’t conform to their leftist ideas) is not even close to meeting this metric. Their attempts to suppress Milo’s speech only empowered his internet-driven platform, just as Twitter banning him did (best thing that could have happened to him).