Most air traffic is IFR. You definitely need radio for IFR traffic in for deconfliction in most of the world. (There's a few exceptions in very remote places, but that's mostly academic.) Even legacy procedural (non-radar) separation assumes you have working radio, and a radio failure is an emergency.
VFR traffic doesn't necessarily need radio, and is not affected by this groundstop.
Right, but isn't a lot of that necessary radio aircraft-to-aircraft, like ADS-B? Obviously not ideal, but I thought IFR traffic could feasibly maintain separation without ATC help.
You don’t need radio for IFR (as the whole point of a flight plan is that if you lose contact you continue to fly your plan). But you kind of do need radar.