A radial just has to be metallic to function. In the country, small-market stations have been known to improvise under certain circumstances. The idea that this happened in the biggest radio market in America is unexpected. Normal radial wire is #16 insulated copper wire, but there's a lot of flexibility in implementation. I've seen galvanized steel wire used, but usually because it's cheap, not because it's ideal.
Now don't confuse radial wires with ground screen wires. The big difference is that radial wires must be insulated from the soil. Ground screen is made of raw copper wire installed just above or below the ground surface. Radial wires need to be specific lengths depending on the frequency of the station. Ground screen is just for grounding, so length isn't very important.
For example, the New York radio station 710 WOR-AM used a set of barbed wire radials. In the late 1970s they had a problem with the existing copper radials were being stolen. This problem continues on today elsewhere with the increasing value of copper scrap. Today's response is video cameras, taller fences and the more traditional use of barbed wire. WOR engineer Orville Sather and consulting engineer Robert Silliman were the innovators that came up with the solution.
They replaced the copper radials with regular galvanized barbed wire. Barbed wire has a scrap value of next to zero. Plus it's very pointy, making it difficult to steal. After a large fire destroyed yet more of the existing radials, they applied to the FCC for a modification. They approved it, but with the requirement that the common point impedance be measured annually. The FCC was concerned about oxidation, because with oxidation resistance increases and performance suffers.
The FCC was right. The WOR-AM tower site is very wet. Like much of northern New Jersey it was built on a landfill. The soil is wet and acidic. The radials didn't last. Eventually they went back to the more mundane copper.