"[A] governmental regime of guidelines which have to be followed, which do not involve private restraints because they would not enjoin any speech in advance. Those guidelines need to be in place so that we don't unfairly surprise the owners of Facebook, or other platforms, or Twitter, but... avoiding the ex-post facto effect of imposing rules after the fact is not the same as violating the doctrine, which basically says you can't muzzle people in advance."