In Debian Buster (now oldstable) I only see 2019.01.17 available:
$ apt list | grep youtube-dl
youtube-dl/oldstable 2019.01.17-1.1 all
$
Same in PureOS 9:
$ apt list | grep youtube-dl
youtube-dl/amber 2019.01.17-1.1 all
$
So in PureOS I had to do a manual install way back in 2018, according to the instructions at youtube-dl: Download Page
Debian Bullseye does have the latest:
$ apt list | grep youtube-dl
youtube-dl/testing 2021.06.06-1 all
$