We have had our free range chickens under and around electric fence for years. I never thought about what would happen if they touched it. So far no problem. Actually it would be amazing if they hadn't touched it at some point. It's very low to the ground in places...
I had thought once about using an electric fence on a switch to keep nighttime predators, and especially early morning dogs at bay. I have been meaning to ask how bad it is for chickens, and you beat me to it. Im also waiting for answers.
I have both electric horse fencing and electric poultry net. If a chicken steps on a downed wire it may get a zap, but mostly, chicken feathers have some insulting qualities that protect the bird. I have not had a problem with chickens and electric fencing.
If they managed to get shocked they will learn to avoid it just like any other animal. They are fairly well insulated though and generally don't get shocked through dry feathers. We use an electric wire to keep them from perching on the watering lines in the hen house. The first week we have new pullets in there, they learn really fast not to perch on the watering lines and after that we shut off the charger. We don't have too many problems after that.