I've seen folks locally riding 32c or even narrower tires on our MUP's relatively well maintained gravel/chat trails. Personally I'm not comfortable with anything narrower than 38, but I'm not fast so there's no advantage to running narrower for me.

A couple of weeks ago I saw a fellow on a drop bar bike with fairly narrow wheels/tires dart up a steep railroad incline lined with rail ballast, those huge chunks of jagged rock. He zipped through that stuff faster and more steadily than I can on my hybrid or mountain bike with 700x42 tires.

I suspect it's as much skill and balance as the tires. They got it. I ain't got it.