Can domestic water lines pass through the return air duct between the floor joist.

Normally I don’t see that but, if the pipe had no joints within the duct, I don’t see a problem with it.

Thanks Larry

Floor or wall return air plenums can not have gas lines or waste cleanouts, and wiring can cross the plenum but not run along the length. Plumbing pipes can either cross or run along the length of the plenum.

Thanks Robert Connors, (you are a wise man). Let me reiterate, so I understand. A water line can go through the floor joists that is used as a return duct.
I have a customer that wants to put a water line through the air duct,so he can eliminate the water line below the return duct and finish his ceiling with dry wall.

I can think of a few reasons not to. Condensation occuring inside the return duct causing mold etc., same condensation causing the duct to rust out and copper pipes will eventually leak.

Is there a code for not having them?

No code to prevent it, however there are flame and smoke requirements for the materials that must be meet.