Can disc brakes be considered safe for road group rides? I know aero bars are considered unsafe for peloton, does this extend to disc brakes whose rotors can be hot and sharp? I'm not talking about races, just casual training rides: is it reasonably safe for others in the group?
I have never ridden in a peloton but I would say they should be not more unsafe than the chain blades that are already on your bike.

While riding, the only parts of your neighbour's bike that you would be able to hit should be the handle bar and maybe their pedals. Everything else is far less outstanding than those two parts and you should collide with your neighbours body before coming even close to say their disc brakes.

So the only real chance to hit someone else's brake rotors might be in case of a crash where you fall on their bike or their bike falls on you. In this case, there are a lot of parts on a bike that could give some bad injuries, for example the chain blades. And you will also hit the ground, so chances are high that you get injured by the fall as well. Adding disc brake rotors (which even are somewhat "shielded" by the wheel itself) might not add significant injury risk in this case. So I would consider them safe in these terms.

Another point that might be worth a thought is their performance. Image the guy in front of a group is the only one to have disc brakes and does an emergency stop. With their bigger brake performance they might decelerate much faster than the ones with the conventional brakes behind them, causing all others to hit him from behind.