In my four years in the Marine Corps I never had to sign anything like that. But that was a different world, the 1960s....
I was in a few years later: Gator fleet in the 70s, took part - off shore - in the fall of Cambodia, and few weeks after that the fall of Saigon, and a while after that the Mayaguez incident. CIA were running all over the place posing as journalists and such, and I saw/heard stuff that never made the newspapers or the history books. And yet, no one came by and demanded I sign a nondisclosure agreement.
I’m curious during which corrupt administration this became SOP.