They did not put that in the readme. The README contains only references to test videos, videos that don't actually exist, and one small-time video (in spanish?) that seems to be an old test video, but I have a hard time seeing for sure.

The source code does contain references to copyrighted videos in the tests, tests intended to make sure that youtube-dl can download the data from videos using the extremely "token" signature scrambling that youtube employs for certain videos. You can see the test cases here:

https://gitlab.com/HacktorIT/youtube-dl/-/blob/master/youtub...