What was signed was almost certainly just a chit to say that you understood your obligations under the OSA. There has never been any requirement to "sign" the OSA. As Mogwi rightly says, if you worked on anything above Secret you were "read in" to the topic, and this then expired when your time of doing work related to that topic came to an end. The only other bit of paper I signed was when I retired, just a reminder that I understood that I was still bound by the same provisions that had applied when working after retirement.
A separate issue is that almost all classified information has a shelf life, beyond which it is either declassified formally, or effectively declassified because it's already in the public domain. One example that springs to mind is WE.177. Shrouded in secrecy for years, until one of the military mags published a chapter and verse info article on the weapons capability, design, even, IIRC, a cutaway drawing, some time before it was officially withdrawn from service. For a time I used one of the S&A key cover tools as a keyring, interesting conversation piece . . .