Technically I don’t think they declassify it, but they can release the contents if they so desire.
Back in the 50’s my dad had a security clearance, and at one point worked on a highly classified project that he said about 10 people, including the President, knew the full details of. A few months after my dad’s involvement with the project ended he heard the President talk about it during a state of the union address.
The next day when he got to work there was a memo waiting for him saying that under no circumstances was he to confirm, deny, or even talk about anything he may have heard the President talk about.
The project remained classified for years after that day because it involved national defense. The President spoke about it to let the USSR know what our capabilities were, but not the specifics of how we did it.
It was the 1950s version of Star Wars. As my dad put it decades after the fact, it was a project to “identify, track, and destroy a hostile satellite”. It was a direct response to Sputnik.
