I actually looked up the 1919 SC commentary from which this cowded theatre thing originated and yeah, it makes no sense so people (and even lawyers) keep quoting this as an example of the limits of free speech in the USA. In reality, there are almost no limits imposed today, and you can absolutely shout fire in a crowded theatre.
it comes down to "can you attempt to change peoples' minds, especially in a way that could cause fear/uncertainty/doubt, when what you're saying is either not true or heavily disputed by other groups or authorities"
can you yell fire when there's no fire, and possibly get people hurt as they run in panic?
can you distribute anti-war pamphlets, when the goverment considers the war to be just, and the soldiers need support that you're inadvertently trying to deny them?
can you yell about fire and brimstone and whip people into a panic about their potential afterlife, when authoritative sources say god is not real and no one should be religious?
should it be legal to convince everyone that some aspect of society has major issues and demands immediate action, potentially causing riots, injuries and deaths, when studies demonstrate that the issue is overblown?