We already recognize that contracts that violate one party's fundamental human rights cannot be enforced because they "shock the conscience", in terms that American jurists use. This article does not include the terms of the non-disparagement clause, or the other terms and payments, so we can't really say whether the clause is vulnerable to being ruled unenforceable by courts. But it's wrong to say that nobody can enter into contracts that constrain their speech. People do that all the time.