We *think* there's an NDA. Or is there?

Someday I will blog about things other than NDAs again but I feel like every time I open the internet there's another story about an NDA. Everyone today was talking about last night's interview of Stormy Daniels on Jimmy Kimmel Live!, which was a bizarre series of answering-questions-with-questions and playing coy and talking around the main issue, which was her alleged affair with Donald Trump in 2006. You can find lots of articles online; here's one that lays it out. Those trying to summarize the interview generally seem to assume that Daniels must be restricted by an NDA, because she could say if there wasn't an NDA, but it's the proving of a negative, basically; the reporters are trying to make sense of the blank space the non-answers leave in their wake. 

It's all had me wondering about the role NDAs played--or maybe more importantly, didn't seem to play?--during the Clinton impeachment. Lots of details about Clinton's sexual harassment history came out during the impeachment, and from my brief research into it, it doesn't seem like there were any NDAs in play. Does anybody have other information about this? How do the number of NDAs around Trump in play today shift our perspective, conversation, and legal analysis?