The problem here were the particular places where github put the tarballs
and their respective signatures, which were out-of-sync.
version=4
-opts="filenamemangle=s%(?:.*?)?v?(\d[\d.]*)\.tar\.gz%youtube-dl-$1.tar.gz%,dversionmangle=s/\+dfsg\d*$//" \
+opts="filenamemangle=s%(?:.*?)?v?(\d[\d.]*)\.tar\.gz%youtube-dl-$1.tar.gz%,dversionmangle=s/\+dfsg\d*$//,pgpsigurlmangle=s/archive\/(\d\S+)\.tar\.gz/releases\/download\/$1\/youtube-dl-$1\.tar\.gz\.sig/" \
https://github.com/rg3/youtube-dl/releases \
(?:.*?/)?v?(\d[\d.]*)\.tar\.gz debian uupdate