The ZDNet reports this is part of a larger action:
"RIAA blitz takes down 18 GitHub projects used for downloading YouTube videos"
and isn't a DMCA 512 takedown:
Although GitHub classified the RIAA letter as a DMCA takedown request, it is not one. As Public Knowledge Legal Director John Bergmayer pointed out today on Twitter, RIAA isn't alleging the library infringed on its rights, but that the library is illegal in itself.
This isn’t really a DMCA request. I don’t see an assertion that youtube-dl is an infringing work. Rather the claim is that it’s illegal per se https://t.co/vQ16nVleCf
— John Bergmayer (@bergmayer) October 23, 2020