youtube-dl -U would be challenging without a download server, but, the issue here is collaboration. Many people have copies of the youtube-dl repo, and many people can build the youtube-dl binary if provided the up-to-date changes. But to develop youtube-dl, they need to be able to continue to collaborate, which is currently at risk due to the DMCA takedown.

If they can continue to share patches over distributed mediums like a mailing list, they can continue to collaborate, and share improvements to the youtube-dl code that many already have copies of.

Possibly not a fantastic long-term solution, but certainly worth attempting whilst fighting it out with the RIAA.

Another important point is that many projects use GitHub as their general communication about issues, support, roadmap, etc. When GitHub suspends a repo, all of it goes away. Whereas Sourcehut is selling the point that your collaboration tools would survive even if they were required to remove your code.