It used to be 50 years ago. But the state where I'm from banned burning trash decades ago. I'm not saying people don't ever do it, but if you get caught you can be fined.

And is this something read somewhere, or a friend of a friend told you in NYC?

This is 100% false that most of rural America still burns trash.

I grew up on a farm, we burnt trash and stopped before I barely remember seeing any fires. Not because we are environmentalist, but because we were told to stop just like everyone else in the state.