Option to download audio before video

[culverit]

Checklist

• I'm reporting a feature request
• I've verified that I'm running youtube-dl version 2020.09.20
• I've searched the bugtracker for similar feature requests including closed ones
  • Relevant issue found #24898 (question, not a request)

Description

I believe issue #24898 is asking if it is currently possible, not if it is theoretically possible.

When running a download the video part is always downloaded before the audio part and then merged.

[youtube] VIDEO_ID: Downloading webpage
WARNING: Requested formats are incompatible for merge and will be merged into mkv.
[download] Destination: VIDEO.mp4
[download] 100% of SIZE in TIME
[download] Destination: AUDIO.webm
[download] 100% of SIZE in TIME
[ffmpeg] Merging formats into "FINAL.mkv"

I am requesting the option to download the audio before the video.

[youtube] VIDEO_ID: Downloading webpage
WARNING: Requested formats are incompatible for merge and will be merged into mkv.
[download] Destination: AUDIO.webm
[download] 100% of SIZE in TIME
[download] Destination: VIDEO.mp4
[download] 100% of SIZE in TIME
[ffmpeg] Merging formats into "FINAL.mkv"

The use case that I have in mind is downloading livestreams (specifically karaoke) after they have ended but before they are privated (usually for avoiding a copyright strike) where the audio is more important than the video but both are desired.
There is often only time to run one youtube-dl command in this small time-frame, so this would be very useful.