In my experience many videos have it, not necessarily ones associated with the RIAA (or perhaps the overzealous[1] content detector just thinks there's some of their content in there), so it's definitely necessary to decode the algorithm for youtube-dl to work on not just RIAA content videos.

IMHO giving the client both the key and the algorithm to decode the content should not count as any form of protection, but the lawyers don't care...

[1] https://news.ycombinator.com/item?id=16075325