> the infohash isn't copyrighted, so it's not illegal information in and of itself. serving the infohash isn't serving the torrent, and serving the torrent is also not serving copyrighted material

I'm of the impression that serving either the infohash or the torrent is considered to violate DMCA. DMCA does not just forbid sharing copyrighted material, but also sharing links to the copyrighted material or generally anything that can help people bypass copyright protections (including software that can decrypt even trivial DRM).

if serving the infohash was a DMCA violation copyright holders would be thrilled to tear down every torrent site in existence. You can't link to the material, but the infohash isn't that either. It's conceptually a link to the torrent. You're partially right about DRM though. Software to directly circumvent copy protections is illegal under DMCA. But talking about the process is still not illegal, and that has led to some really clever workarounds like the DeCSS Haiku (https://en.wikipedia.org/wiki/DeCSS_haiku)