Although American law moved toward a vastly more liberal idea of free speech during the 20th century, you should keep several things in mind when thinking about that trend. First, the trend was not unbroken. Supreme Court cases generally enlarged the area of protected speech, but not always. Second, there are principles and arguments in some Supreme Court cases that have acquired a somewhat doubtful status because of later cases. Third, the Court has never said that all speech is protected.
Political speech receives the highest level of pretection.
Commercial speech and obscene speech have never been given the same degree of protection. Fourth, there was nothing inevitable about the more liberal interpretation of the First Amendment. Most countries, including most wealthy democratic countries, have never developed as liberal a view as the United States. As Samuel Walker points out in his history of hate speech, our own results were influenced greatly by organized argument and pressure. The Supreme Court has not always sided with the advocates of greater freedom.