[Campion]

The television camera system on the Apollo 11 lunar module was a non-standard, low-res, "slow scan TV" device, to conserve weight and bandwidth. The signal from that system had to be upconverted for broadcast TV. The mag tapes of the original slowscan TV signal were accidentally erased and recorded over. Only the (badly) upconverted version survives.

I believe that's what you're remembering. I'm unaware of any other large-scale loss of worthwhile Apollo artifacts.

Clementine was a joint effort by NASA and the DOD, specifically the SDI program. I'm not able to find corroboration that there are large numbers of Clementine images that remain classified, but if there are, it's because the government wants to keep their imaging capabilities a secret.

[PIF]

Clementine tech is 1994. There is nothing technical image-wise that has not been superseded many times over since then; therefore, on technical grounds there is no reason to keep it classified. It was the US Navy that funded the lunar mapping project from its budget; NASA and DOD were secondary users.