I just installed it from archive.org using the url homebrew uses. It's a simple matter to run brew install -d youtube-dl plug the url that into archive.org, download the archive, cp it to the dl location (minus the .incomplete) that brew expects, and then run it again without the -d so it thinks it is already downloaded.

I wouldn't have installed if they hadn't done this, and I'll probably never use it, but this rubbed me the wrong way.