No bits are *truly* made of silver, except for pure novelty. A silver bit would obviously be very expensive and difficult to maintain. The bits marked as silver are usually actually made of “German Silver”, which is a copper alloy of up to 60% copper mixed with zinc and nickel.
Like other copper-based bits, these have a sweet taste that encourages good salivation, but the nickel in German Silver can cause issues if your horse has an allergy to nickel that could cause mouth sores.