My big birds like to dig through the leaves, they think it's fun. I tried burning leaves once... the firefighters and ambulance cameNot an expert and guessing...
In the fall I gather great amounts of leaves from my property and put them in the run, my flock goes nuts and spends days tromping them down and eating the bugs that come along with them. Also adds very beneficial organic material to the run.
The leaves are on the trees and, it seems to me, very unlikely to have been sprayed with anything unless they come from fruit trees. Guessing most are very safe for whatever use you have for them.