Depends. 80% sure the last place I worked at had Non-Disparagement on hire. They were not primarily an IT company, and their primary pool of new recruits for their core business was adults fresh out of college. I don't think it's entirely coincidental that there was a -lot- of scary language in the agreements/handbook that seemed geared towards making sure people were afraid to talk about the problems the organization had.
> As always HR is not your friend.
My last day on that job, I had to do an exit interview. During said interview, the HR employee asked how I was doing that day, my answer was, "Considering this is the first time you folks are pulling me into a room since I got here, not bad!"