BTW, they also did allege infringing material. Whether or not that material's copyright is in fact infringed upon by youtube-dl is not for GitHub to decide. (And, in my non-lawyer opinion, a US court would probably say it is, if this ever makes it that far.)
https://github.com/github/dmca/blob/master/2020/10/2020-10-2... says:
> The clear purpose of this source code is to ... reproduce and distribute music videos and sound recordings owned by our member companies without authorization for such use. ... We also note that the source code prominently includes as sample uses of the source code the downloading of copies of our members’ copyrighted sound recordings and music videos, as noted in Exhibit A hereto. For example, as shown on Exhibit A, the source code expressly suggests its use to copy and/or distribute the following copyrighted works owned by our member companies: • Icona Pop – I Love It (feat. Charli XCX) [Official Video], owned by Warner Music Group • Justin Timberlake – Tunnel Vision (Explicit), owned by Sony Music Group • Taylor Swift – Shake it Off, owned/exclusively licensed by Universal Music Group