I am honestly confused with why everyone considers non-disparaging agreements bad in all contexts.
People often end up hating each other, eg sometimes bad break-ups happen. I do think that “I am not going to bad-mouth you, you’re not going to bad-mouth me” is a sensible fair thing to do that allows people a) move on with their lives without a fear of retaliation, b) allows more honest relationships when they work because you don’t have to be constantly collecting “evidence” for potential future defense?
And, the same way this makes sense in relationships, I see it making sense in employer-employee relationship?
I would say it to be quite unusual to not be allowed to say that you are under NDA.