[Rockingham]

In the main you are correct about the extent of declassification and the resulting mass of documents from WW II that are yet to be reviewed by outside researchers. Yet there are some documents that have disappeared, with others withdrawn from the National Archives by secretive agency owners like the NSA and CIA, and large tranches of files are still held back at the request of friendly foreign governments and intel agencies.

For example, Bruce Lee's path-breaking Marching Orders recounts numerous discoveries he made in the records of of the intelligence analysis of high-level Japanese and German codes during WW II. Yet he was denied access to some records and was told that they would never be made available to researchers.

[GreyFriar]

Intelligence files were not declassified by General Eisenhower in 1947, along with files on the Manhattan Project, the Pozit fuse, radar, and other technical advances. He ordered all the non-sensitive non-technical files. You are correct about those records that have not yet been declassified, but those records are post WW2.

Files relating to codes have an indefinite lifespan. Just like intel files on people here in the US who reported their neighbors as suspected Axis spies and saboteurs.