What FB in NY said. Most of the old ones were way more ductile. We used to ride our 'bars until they drooped into completely different bends. A sag of 2 degrees at the bar clamp meant you rode a lot. When it was more than three degrees it was time to replace your bars. Unless you were cheap. I have seen and experienced quite a few brittle failures of stiff handlebars. The old ones just drooped and drooped some more.

Lots of modern stems break too. Only one I've broken was modern.