The JSON response changed and the extractor needed to be updated in order to gather the video IDs.
Like: -f " bestvideo+bestaudio/ best "
For iframe URLs
@dstftw explained in https://github.com/rg3/youtube-dl/pull/9918#issuecomment-228625878 that extractor names are supposed to be unique. @dstftw has fixed the two offending extractors, and here I add a test to ensure this does not happen in the future.