Do you recall anything about where those herds are, because I’ve only seen research showing that all treated herds have identifiable cattle DNA, i.e., https://www.nature.com/articles/s41598-022-09828-z
I was told when I worked at Philmont Scout Ranch [1] that they had a pure herd.
Other than that, I know Steven Rinella listed a few pure herds in his (excellent) book [2] on the American Buffalo, but I'd have to dig it out to find them for you.
"Wind Cave and Yellowstone National Parks are the only two federal herds to have population sizes large enough for sufficient testing. Both herds show no evidence of cattle introgression."
