Keeping the same safe houses or reporting structures in place for 60 years seems like it would be terrible OPSEC. The IC is very good at coming up with plausible sounding explanations for why they have to keep secrets that the masses will believe without question but that don't stand up to deeper reflection.

I suspect the remaining redactions are because they're waiting for certain people to pass away. Probably not major players but there could be people who were in their 20's or 30's in 1963 who had some knowledge of what happened. Once they've all died off it will be forever impossible to prove anything because anything really damning would never have been written down or the records would have been destroyed years ago.