Add to this that the original author recently posted a story about the origins of the youtube-dl script admitting it was designed to do:- download YouTube videos and name the downloaded files appropriately.
https://rg3.name/202011071352.html
Under DMCA, neither writing a script like youtube-dl nor using it is prohibited (making an unauthorised copy of a video could be fair use).FN1 Section 1201 however prohibits distributing the script to others. Thus, the author of the script who "releases" (distributes) it is not necessarily the only one who might be violating the DMCA. Any recipient of the script who distributes it further, e.g., Microsoft, could be violating the DMCA as well.
FN 1. Section 1201 prohibits distributing technology that is designed to circumvent either "access controls" and/or "copy controls". Similarly, the act of circumventing "access controls" is prohibited. However, the act of circumventing "copy controls" is not explicitly prohibited. Making unauthorised copies, e.g., downloading YouTube videos, is subject to the defense of fair use. It is arguable that youtube-dl is only designed to circumvent "copy controls". As others in the thread point out, there are generally no "access controls" on YouTube videos, e.g., password protection. There could be exceptions. If youtube-dl is designed to circumvent geographic or age restrictions, would those be considered "access controls".
Aside from DMCA concerns, Google's Terms of Service for YouTube would appear to prohibit use of youtube-dl:
"The following restrictions apply to your use of the Service. You are not allowed to:
1. access, reproduce, download, distribute, transmit, broadcast, display, sell, license, alter, modify or otherwise use any part of the Service or any Content except: (a) as expressly authorized by the Service; or (b) with prior written permission from YouTube and, if applicable, the respective rights holders;
2. circumvent, disable, fraudulently engage with, or otherwise interfere with any part of the Service (or attempt to do any of these things), including security-related features or features that (a) prevent or restrict the copying or other use of Content or (b) limit the use of the Service or Content;
3. access the Service using any automated means (such as robots, botnets or scrapers) except (a) in the case of public search engines, in accordance with YouTube's robots.txt file; or (b) with YouTube's prior written permission;"
https://www.youtube.com/static?template=terms
Would these TOS be enforceable if challenged. #1 makes no allowance for fair use. What do you think.