Copper supply plumbing touches steel drain pipe. What can I do?

The best answer would be to re-route the copper pipe around the steel pipe. If you don't have a tubing bender to fit the pipe, you can get a piece of soft copper and bend it by hand to get around the steel pipe. If you don' t have access to soft copper, you can use a 90 degree elbow with a 90 degree street elbow to jog around the interference. The use of too many elbows is not the best system, so use good judgment. Using a shim or even wrapping the copper pipe with pipe wrap tape is not a long term solution.

Bend the copper pipe or use a cedar shim. Either a bad install or the walls have shifted. Pipes do not normally touch except for the previous 2 reasons. Proper fix is to reroute it around the steel/iron/lead pipe.

In new construction, thin sleeves of polyethylene (like trash bags are made of) often are used to prevent 2 pipes from touching. It doesn't take much, you just need to prevent electrons from one pipe from going to the other (as water flows through, it creates tiny electrical currents which will try to electroplate one pipe with metal from the other), and it must stay in place. I'd say wedge a slab of thin foam rubber between the two and duct tape it to hold it in place.

I would try to wrap the copper with foam insulation sleeve.

A shim will work, even a piece of cardboard which is always available on a job site.

Garden hose. Cut whatever length you need and slice down one side, snap onto whichever is the easiest.