It seems to save a lot of hassle to just say "Note:" instead, rather than an obscure acronym that already has a non-obscure, short equivalent.
Reminds me of a forum I was on, where people would edit posts and preface the edit with "ETA:" (edited to add). But virtually every time someone had to ask what it meant, and eventually people started using "Edit:".
Perhaps you are right - my 'problem' is that my parents are of an age were they studied Latin at High School, and as a result some of their education rubbed off on me; I no longer remember where I learned 'NB' (and other Latin miscellanea). Try living with parents who liked making witty and snarky comments in Latin about one and one's brother when one was trying to survive life as a nerdy teenager! :)
It's more like a class signifier. People who have certain educational backgrounds, or read a lot of certain types of writing, are more likely to use it.
Everyone in the UK would know what it means. I doubt it is particularly common in legal documents. Probably something to do with having a decent proportion (>0.001%) of the population educated in the classics...
Reminds me of a forum I was on, where people would edit posts and preface the edit with "ETA:" (edited to add). But virtually every time someone had to ask what it meant, and eventually people started using "Edit:".