It is interesting how the taboo against going barefoot can be local to particular regions and to particular times.
In the 60's in Hawaii, being shoeless was how my brother and I played outside and ran around. Of course, we had shoes for really rough terrain - hot pavement, lava rock, etc., but as far as I could tell, no one cared at all if someone was running around without shoes. People might have recognized us as being haole, but there never was a word said about not wearing shoes.
This shoeless behavior stopped in the Iowa, though, since bare feet and gravel roads are - in my recollection - not compatible. No one in my part of western Iowa used the term hillbilly as a derogatory term as far as I ever remember hearing, but redneck was prevalent as a putdown. I have never associated the term 'redneck' with being barefoot though. The term 'redneck' seemed to float around without any attachment to a particular set of characteristics. (Without looking up the term's origin, I cannot say to what group the term 'redneck' is supposed to refer.) It seemed to be synonymous with simply being ignorant.
I have heard the term 'farm boy' used against Iowans. However, since an awful lot of Iowans are from rural areas, this construct has no effect - it seems redundant and thus meaningless. Maybe this term is the closest in spirit to the term 'hillbilly', but 'farm boy' has never gained currency as a putdown.