Something everyone is signing or your raise gets withheld...
My company has a lot of bad (and well-earned) reviews on Glassdoor and stuff. So, rather than try to make it a better place to work...they just make you sign a document that says that you won't write something negative. I imagine that after my termination, this document won't be valid anyhow...but sure...I won't make fun of your wee-wee on TV or whatever...
I believe you work in New York? I took a quick look at our restrictive covenant manual. The non-disparagement clause is most likely valid and enforceable, and it would be even if they fire you. It's a valid contract.
Did it include other terms, such as a non-competition clause or restrictions on working for competitors?