What would a drywall screw from 1972, inadvertenly drilled into a copper water line, look like today?How rusty?

Yes galvanised screws were used in the 70's. and they are now, otherwise as you know they would rust. Copper does not rust, so a galvanised screw through a copper pipe, in the water did not rust, and over a long period of time, says a lot for the quality of the screw