Technically, no. Other metals are also amenable to anodization - titanium, magnesium, zinc. It’s essentially a pre-oxidation of the surface. But it’s true that of the anodized metals, aluminum is most common for whistles and other consumer products.
I have half of one (kept the top, gave away the bottom) where I thought the body was steel - very heavy, nickel plated and felt like chairleg metal. Low E flat.
I now have it pushed on the end of six feet of plastic plumbing pipe so it works as an overtone whistle. Much better.
A local got a Howard head from someone who had slammed the body of his Howard low D in a car door. He cut off the top of a Shaw low D and grafted the Howard head onto it. Works pretty well.