This would likely become the employment equivalent of those fine-print "Buy ordering from our website, you agree to not write disparaging reviews of our product

This would likely become the employment equivalent of those fine-print "Buy ordering from our website, you agree to not write disparaging reviews of our products." policies on various web outlets. By all accounts non-enforceable, but more than likely would just be used as a bullying tactic against people who lack the funding to take it to court.