You never want to put Thompsons on any wood that will be used in a garden as the chemical (like most) will leach into the soil.....then into the food.

To answer the OP's question, I have never seen or heard of cedar fence planking treated with any chemicals or pressure treated so I called my friend who is the manager of a 3rd generation family owned lumber yard, he says it has never seen it come through their yard. Given the natural resistance of cedar from rotting does not make it cost effective to treat.

In my own garden, we have cedar lined beds that are now 6-7 years old and have no signs of rot and we are in the rain prone PNW area....