Actually Schenck v. United States was partially overturned and severely limited 53 years ago, so that hasn't been correct, technically, for some time. iow, you probably can shout fire in a crowded theater, but it depends on the circumstances whether you'll be arrested, whether you've presented a clear and present danger, whether you caused a stampede, whether anyone was injured or killed. But if you are arrested, it won't be a free speech case. The charge will be disorderly conduct, or negligent homicide, or some other charge. Justice Holmes' quote has sort of taken on a life of its own as a determination of what speech is not protected, but really there's no law against it. FWIW, I'm not advocating for it, just being pedantic.