I know certain files by the US government are sealed for a certain time. Like I think census data is sealed for 70 years.
But is there anything now being held sealed in perpetuity? Like say a defense plan from the Civil War or something really old. Or will everything get released eventually?
How would we know?
There’s the Freedom of Information Act, but that only works if you trust the government to properly classify its documents and to act in good faith even when a release will put itself in a bad light, which is both against human nature and against the interests of the Powers That Be.
If the government wants to hide something from you and you’re only a citizen, it’s pretty likely you’ll never hear about it.
nm
I’m sure Rose Mary Woods has a few under the stairs. 
Yes, for instance there’s [redacted], the [redacted] commission report, and the [redacted] inquiry.
I believe that some individually-identifiable materials, such as tax returns or personnel files, are never made public, but they’re probably not kept forever either.
Hmm. Nicolas Cage was in that documentary a few years ago about the President having a book that no one else could look at…
The Kennedy files are an obvious answer. Weren’t they due for release a few years ago? Then the date got pushed up again.
Isn’t declassification or length of classification a modern idea anyway? Earlier documents were just stored and no one was permitted to see them and they were forgotten until rediscovered by historians centuries later? I suppose that is what will happen to the surviving types of documents you mention.
Nope. As your link says, in the early '90s Congress created a review board for the express purpose of making a lot of those documents public, and that’s what happened. The National Archives says almost all of those documents are now public, and they have been for 15 to 20 years (not including stuff that was public before that). The site says, and I apologize for the unnecessary modifiers, “With a very few exceptions, virtually all of the records identified as belonging to the Kennedy Collection have been opened in part or in full.”
The National Archives says that’s a few years away:
In our state, certain legislative records (basically proposals for legislation that never got to the introduction stage) are retained in perpetuity, and can only be accessed by legislators or legislative staff. The are never opened to the public.
Proposals to open these records after, say, fifty years have so far gone nowhere.
I remember seeing Senator Barry Goldwater on television in 1971 after the Pentagon papers were released. While he denounced Daniel Ellsberg and the New York Times for publishing, he did say the government kept too many papers secret. He said that Pentagon officials had told him that there were still papers from Custer’s Last Stand (1876) still classified.