The space armament treaty says: no nuclear, biological or laser weapons in space. but kinetics...
Why in the hell would anyone bother though? First you got to launch all that mass into a stable orbit. Then you got to assemble the delivery system with the mass as it's most definitely too heavy for a single launch. Then you need fuel to deorbit the mass when you launch because things in stable orbits tend to want to stay there. Then you wait for the mass to deorbit because we couldn't afford to send enough fuel for a rapid deorbit. Also wait to launch till optimal trajectory for your target is achieved and hope that it's not too far side to side from your orbital path because that means even more fuel to deorbit. Also anyone with a halfway decent telescope sees your weapon just sitting there in orbit not to mention being assembled so now it's got a massive target painted on it at all times and is an easy casualty of first strike.
Or we could use the icbm's that are a proven tech, easier to hide though that's not foolproof, can be made mobile, much much more numerous, easier to protect from attack, much cheaper than launching tons of solid metal into orbit, and can strike anywhere on the globe within an hour.
I mean, launch costs are going down over time. It might make much more sense to put many warheads up with a reusable system like falcon (~3000$/kg) than it does to maintain a fleet of necessarily single-use systems like minuiteman (7,000,000$/~300kg = 23,300$/kg). You might well be able to put four warheads (with an equal mass of de-orbiting propellent into orbit - ~1.7 km/s given solid fuel) for the same cost as one ICBM.