The most common usage is in reference to a valuable metal used in electronics and catalytic convertors. It was discovered in 1803.
However, an idiomatic usage is for something that affords protection. It comes from “Pallas,” which was another name for the Greek goddess Athena. A statue of Athena was believed to have the power to protect the city of Troy.
Merriam-Webster provides some examples of this usage:
…this political persecution being, somehow or other, the grand palladium of our liberties.
…lend authority to the view that the true palladium of every people is its language…
I had to search for “palladium idiom” to find this usage, so, clearly, a reference to the metal is much more common.