There are some that are all metal except for the fipple plug (like Alba), and variations close to that (Burke, Anak, many more).
1)
the Skellig low whistle is all aluminum (but is tunable). Ask Nick for a non-tunable High D Skellig, or an improv model high D. He's a creative genius. Website >
https://www.theirishwhistle.com/models2) can't find one since the Chieftain high D Thunderbird was stopped > search online for used ones, and email Phil Hardy at Kerry Whistles to tell him you'll buy one if he's got an old one somewhere, even if it's scratched and the dog plays with it.
https://kerrywhistles.com/3) Carbony is an option you might consider if you're trying to get away from the tone from brass, plastic and delrin.
4) I went to the website of Goldie but none of the photos show the fipple plug. All aluminum otherwise, including mouthpiece.
This makes me wonder if the tone of a whistle could be altered to any considerable degree by covering a plastic fipple plug within the tube area, with a sheet of aluminum foil. Or some such process. If it's far more convenient to use plastic as a fipple plug material, then if the sound is made too different, remetalize the surface of the fipple plug after it's installed. Always a workaround.