It doesn't have anything to do with classification. It is ultimately an unworkable idea. I worked in space-based weapons and know the problems. For one thing, the terminal effects are entirely a matter of conjecture. The assumption is that all the kinetic energy converts to heat immediately. Except when this happens on a terrestrial level (tank penetrator round) that is not what happens. The rod simply penetrates as far as it can. No subnuclear explosion, and a God-awful mass to haul into space, with associated de-orbiting propulsion system. There are other ways to do any job worth doing.

Oh. I had a critical nuclear weapon design information (CNWDI) clearance and almost got read into a Q clearance. But the bright idea I would have been working on was found to be a dud, so the clearance was dropped. Not a "Rod from God," but no doubt something that someone else will bring up as a hush-hush secret weapon.