I've haven't had any steel road bars in a long time. But the only uncomfortably stiff bars I've had were aluminum, some very cheap MTB bars that must've been way too thick-walled, they felt like hitting a fastball with a wooden bat turned the wrong way!

I do have some carbon road bars that I like quite a bit, but those require a stem also on vintage bikes. I have a couple of old steel bikes with 31.8 clamp stems just because I like modern shaped bars with a kinda short reach, flat ramps, and fairly shallow drop. These are bikes I take on long rides. On the old Legnano, Schwinn, and Scapin I have old school bars but wouldn't be super happy riding those for 60-100 mile rides.

I also have some Surly Open bars, which are steel, which I find quite comfy.