$ streamlink -l debug 'youtube.com/watch?v=yZ7CWjuMCe8' best
[cli][debug] OS:         Linux-5.12.4-2-git-x86_64-with-glibc2.33
[cli][debug] Python:     3.9.5
[cli][debug] Streamlink: 2.1.1+16.g034173d
[cli][debug] Requests(2.25.1), Socks(1.7.1), Websocket(0.58.0)
[cli][debug] Arguments:
[cli][debug]  url=youtube.com/watch?v=yZ7CWjuMCe8
[cli][debug]  stream=['best']
[cli][debug]  --loglevel=debug
[cli][debug]  --player=mpv
[cli][info] Found matching plugin youtube for URL youtube.com/watch?v=yZ7CWjuMCe8
[plugins.youtube][debug] Video ID from URL
[plugins.youtube][debug] Using video ID: yZ7CWjuMCe8
error: Unable to open URL: https://youtube.com/get_video_info (404 Client Error: Not Found for url: https://www.youtube.com/get_video_info?video_id=yZ7CWjuMCe8&el=detailpage)

$ streamlink -l debug 'youtube.com/watch?v=yZ7CWjuMCe8' best
[cli][debug] OS:         Linux-5.12.4-2-git-x86_64-with-glibc2.33
[cli][debug] Python:     3.9.5
[cli][debug] Streamlink: 2.1.1+17.g105cc24
[cli][debug] Requests(2.25.1), Socks(1.7.1), Websocket(0.58.0)
[cli][debug] Arguments:
[cli][debug]  url=youtube.com/watch?v=yZ7CWjuMCe8
[cli][debug]  stream=['best']
[cli][debug]  --loglevel=debug
[cli][debug]  --player=mpv
[cli][info] Found matching plugin youtube for URL youtube.com/watch?v=yZ7CWjuMCe8
[plugins.youtube][debug] Video ID from URL
[plugins.youtube][debug] Using video ID: yZ7CWjuMCe8
[plugins.youtube][debug] get_video_info - 1: Found data
[plugins.youtube][debug] MuxedStream: v 299 a 251 = 1080p60
[plugins.youtube][debug] MuxedStream: v 302 a 251 = 720p60
[plugins.youtube][debug] MuxedStream: v 135 a 251 = 480p
[plugins.youtube][debug] MuxedStream: v 133 a 251 = 240p
[plugins.youtube][debug] MuxedStream: v 160 a 251 = 144p
[cli][info] Available streams: audio_mp4a, audio_opus, 144p (worst), 240p, 360p, 480p, 720p, 720p60, 1080p60 (best)
[cli][info] Opening stream: 1080p60 (muxed-stream)
[stream.ffmpegmux][debug] Opening http substream
[stream.ffmpegmux][debug] Opening http substream
[utils.named_pipe][info] Creating pipe streamlinkpipe-93906-1-8141
[utils.named_pipe][info] Creating pipe streamlinkpipe-93906-2-5957
[stream.ffmpegmux][debug] ffmpeg command: ffmpeg -nostats -y -i /tmp/streamlinkpipe-93906-1-8141 -i /tmp/streamlinkpipe-93906-2-5957 -c:v copy -c:a copy -map 0 -map 1 -f matroska pipe:1
[stream.ffmpegmux][debug] Starting copy to pipe: /tmp/streamlinkpipe-93906-1-8141
[stream.ffmpegmux][debug] Starting copy to pipe: /tmp/streamlinkpipe-93906-2-5957
[cli][debug] Pre-buffering 8192 bytes
[cli][info] Starting player: mpv
[cli.output][debug] Opening subprocess: mpv --force-media-title=youtube.com/watch?v=yZ7CWjuMCe8 -
[cli][debug] Writing stream to output
[cli][info] Player closed
[stream.ffmpegmux][debug] Closing ffmpeg thread
[stream.ffmpegmux][debug] Pipe copy complete: /tmp/streamlinkpipe-93906-1-8141
[stream.ffmpegmux][error] Pipe copy aborted: /tmp/streamlinkpipe-93906-2-5957
[stream.ffmpegmux][debug] Closed all the substreams
[cli][info] Stream ended
[cli][info] Closing currently open stream...