Historical clue formulations

• On a need-to-know basis — 2021-04-15
• Upper-level security clearance status — 2020-08-14
• Highly classified — 2017-01-16
• Highly classified — 2016-10-24
• Phrase often stamped in red — 2015-02-21
• Classified — 2013-11-08
• Known to next to none — 2006-06-03
• Like the Manhattan Project — 1996-09-21