Hi Rob. I would recommend going with what the docs say and use TfHashAppend.
OpenUSD used to use boost’s hashing methods, which uses ADL on hash_value. At some point, TfHash was introduced and for backwards compatibility, it supported both hash_value and the specific TfHashAppend. std::hash support was added when Pixar switched to C++17 (Pull Request #2781 · PixarAnimationStudios/OpenUSD).