90-year-old Cryptanalytic Efforts Must Stay Secret, says NSA
It's easy to imagine scenarios where this continued classification could be appropriate.  Remember, classification doesn't just attach to facts; it also attaches to how that information was obtained.If, for example, the censored paragraph was "a US agent placed within the Soviet Communist Party participated in the design of Soviet ciphers and deliberately weakened them", and that agent continued working in the USSR for several decades, then it could only be declassified if everything that agent had ever been involved with was no longer sensitive.