That term used to be an obsolete security term. In my time it joined NOFORN as a complete waste of ink. I remember one comment by a general officer that "this should be formerly restricted data" because the author just described exactly how to make a nuclear bomb; something he was familiar with since he used to make them. I didn't see any answer by anybody above in the chain of command but when you know a guy who truly does know how build atomic and hydrogen bombs, you don't get lost on the significance of the evidence.
The only thing keeping them rare is the absence of access to fissionable material. Based on evidence from the Air Force about mishandling fusion bombs I really am kind of surprised DC, Detroit and NYC have not blown up yet.
In time perhaps. I wouldn't want to live there. It's just a matter of time.