"Both parties agree that for two years from the signing of this agreement they will not make disparaging remarks about each other to third parties."
Not so long ago this would have represented some kind of wild fantasy about business relationships that was featured in the HARVARD LAMPOON. That was then.
Now there is no social contract between those writing the check and those doing the work.
There is social media to air one's beef. Do that in a provocative way and it can go viral. The whole world will bear witness to what pond scum that client was.
In addition, given that we're all overwhelmed with stress, it feels right to release the poison.
Those of us who think like lawyers will notice that the provision does not ban making disparaging remarks about each other to each other. Incivility is jolly okay. The client or I will call each other pond scum to each other's faces.