Best bitrate they had was 192kbps for HD. I don't know if that has changed since they introduced 4K. Vimeo has much better video and audio encoding compression at least, you get 320kbps.
For the Rob Scallon video posted above:
$ youtube-dl -F
https://www.youtube.com/watch?v=PtwXdKkO75E
[youtube] PtwXdKkO75E: Downloading webpage
[youtube] PtwXdKkO75E: Extracting video information
[youtube] PtwXdKkO75E: Downloading DASH manifest
[info] Available formats for PtwXdKkO75E:
format code extension resolution note
140 m4a audio only DASH audio 129k , m4a_dash container, aac @128k (44100Hz), 3.13MiB
171 webm audio only DASH audio 141k , audio@128k (44100Hz), 3.16MiB
141 m4a audio only DASH audio 255k , m4a_dash container, aac @256k (44100Hz), 6.21MiB
"webm" implies Opus.
What encoder YouTube uses for their AAC streams is still anyones guess, but it's been like this for some time now.