I would suppose that whether a copper shovel would be a workable tool in a given soil would certainly depend upon how hard the soil is.
In my local sand, I would expect it to be just fine. (My double-dig book talks about it being really tough to get down to their recommended 2 feet soil loosening depth in the first year, BUT I -- a particularly petite and not especially strong woman -- can dig out a hole that deep by just keeping at it and never going so fast that I get winded. It is a long task, but not especially hard.)
In clay soil, you are probably right that the softness of the metal would be a real issue.
My memory is that in Ancient Egypt (like the time of the Pharaohs and the Pyramids) copper was the metal that they made all their tools out of. It was their technological advantage over the surrounding populations.