Elemental copper is an antibacterial. In small quantity, its not only safe, but necessary.I remember reading that old copper pennies (Wheaties?) or small copper fittings help to discourage algae. Not sure if it’s true or safe for chickens.
That said, there is very very little copper left in a modern penny, a lot of things much less palatable in a penny (or on it), and - once a layer of surface oxidization forms - no copper is exposed to further retard bacterial growth. The large cuantity of zinc in the penny - another trace metal - is similar to that used to protect the galvanized bucket.
https://www.usmint.gov/learn/kids/coins/fun-facts/category/penny
You do know what happens when two dissimilar metals are connected by water or make contact in a circuit, right???
Not being a qualified metalurgist, this is a high school chemistry experiment I would prefer not to routinely practice in my bird's drinking water. Its enough that I collect rainfall off of a galvanized roof and store in 275 gal poly totes (THICKLY painted to reduce UV penetration. Drained and cleaned yearly - and is that ever a pain!)