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.
During the early years of WW II, the British shifted many archival file to storage facilities secure from the Luftwaffe's bombs. The process unearthed the previously lost records of Britain's relationship with Benedict Arnold, which then lead to a revealing academic work on the subject. That marked the end of various defenses of Arnold as misunderstood and unfairly maligned by jealous enemies in the Continental Army.
Despite efforts at suppression, secrets tend to leak out. In the 1980s, a college student collected bits and pieces of information about nuclear bomb design from various archives. His resulting thesis was so revealing that it was classified and he had a long fight before he could publish it and his resulting book. The essential secrets tuned out to be relatively straightforward applications of what are now well-recognized principles of nuclear physics.
As a rule of thumb, secrets tend to come to light officially only when scientific and technical progress or the passage of time make them almost solely of historical interest. The world at large gets to know secrets only when the secrets no longer matter very much.