Not sure if I phrased this question correctly. What I mean by this is, I think pretty much every "government secret" before WWII has been declassified to the best of our knowledge? I'm wondering if there is an exact year we can go to where everything that happened before that has been fully declassified to the best of the government's ability.
By "Government Secret" Do you mean written records still extant today?
There are a good many secrets (i.e. intelligence networks from the American Revolutionary and Civil Wars) that former government officials never committed to written records or perhaps destroyed the records without ever making them public.
This is actually more of a U.S. legal and policy question, so I feel confident I can jump in here. Mods, I won't be offended if you feel otherwise.
Here is the current Executive Order providing policy guidance to the U.S. Government on classified materials: https://www.whitehouse.gov/the-press-office/executive-order-classified-national-security-information
Here is the most relevant section:
Sec. 1.5. Duration of Classification. (a) At the time of original classification, the original classification authority shall establish a specific date or event for declassification based on the duration of the national security sensitivity of the information. Upon reaching the date or event, the information shall be automatically declassified. Except for information that should clearly and demonstrably be expected to reveal the identity of a confidential human source or a human intelligence source or key design concepts of weapons of mass destruction, the date or event shall not exceed the time frame established in paragraph (b) of this section.
(b) If the original classification authority cannot determine an earlier specific date or event for declassification, information shall be marked for declassification 10 years from the date of the original decision, unless the original classification authority otherwise determines that the sensitivity of the information requires that it be marked for declassification for up to 25 years from the date of the original decision.
(c) An original classification authority may extend the duration of classification up to 25 years from the date of origin of the document, change the level of classification, or reclassify specific information only when the standards and procedures for classifying information under this order are followed.
(d) No information may remain classified indefinitely. Information marked for an indefinite duration of classification under predecessor orders, for example, marked as "Originating Agency's Determination Required," or classified information that contains incomplete declassification instructions or lacks declassification instructions shall be declassified in accordance with part 3 of this order.
Most classified information is automatically declassified after ten or twenty-five years. That can be extended in some cases, but you'd need to read the entire policy to understand the various reasons and the processes to do that. A couple notable exceptions include when declassification would reveal the identity of a confidential human source or a human intelligence source; or key design concepts of weapons of mass destruction. In addition, agency heads can petition to keep other information classified. For example, if for some strange reason there is an old school cryptological method or a bit of intelligence gathering trade craft that was developed decades ago and is/still could be used then it will likely remain classified until it becomes obsolete (I'm sure some actual historians can jump in with examples of sources or methods of intelligence gathering that remained classified for a surprisingly long time because they remained in use).
Overall, there is good chance that some information from the Manhattan Project remains classified. That WMD information may remain classified practically indefinitely as long as not every nation has a nuclear weapon. There are probably a handful human intelligence sources dating back to the early 20th century who's identities are still classified, though those should eventually be declassified. Portions of a few old warplans might still be classified if certain elements carried through to newer revisions throughout the years (e.g., key defensive points originally identified in an old plan may still be relevant today and included in the current iteration of a warplan).