It has, apparently, never been enforced. Largely, I guess, because it didn't need to be and it would likely be overturned.
"However, the Supreme Court of the United States has not ruled on the act's constitutionality. Despite that, no administration has tried to enforce it. The provisions of the act outlawing the party have not been repealed. Nevertheless, the Communist Party USA continues to exist in the 21st century."
Interestingly, there actually is (as in, still is!) a law making the Communist Party and membership of it illegal: https://en.wikipedia.org/wiki/Communist_Control_Act_of_1954
It has, apparently, never been enforced. Largely, I guess, because it didn't need to be and it would likely be overturned.
"However, the Supreme Court of the United States has not ruled on the act's constitutionality. Despite that, no administration has tried to enforce it. The provisions of the act outlawing the party have not been repealed. Nevertheless, the Communist Party USA continues to exist in the 21st century."