I can sort of see the point if the finger of suspicion is pointing at your cold-war enemy. If they’re trying out some new technology, do you want them to know you’ve seen it, or is it better for you to let them think it’s somehow invisible (stealthy).
I just don’t believe that everything is handed over willy-nilly just because of some FOI request or someone decides a set number of years has elapsed.
The Pitlochry Harrier incident remains unexplained though. Given the experts concluded it was Harriers seen beside the unexplained shape, how come there were none reported to be flying at the time? Surely that in itself would have been easy to follow up?