YouTube used to allow multiple audio files for one video, ...you could have a dedicated stereo track + a 5.1 track. Some YouTube channel owners used this as a way to create a TEST-VIDEO, ...so that users could check to see if they were actually hearing 5.1 or not. One track was "You are listening to the stereo track" and the other was "You are now streaming the 5.1 track"
With the way I mix, on some songs, the stereo mix-mixdown that YouTube performs is horrible and causes weird phase issues with what was in my Center channel and they spread it to the stereo pair. Because of this and the fact that sometimes I spend 3 days working on a single song, I wanted to upload my own stereo mix embedded into my standard 5.1 videos, ...just in case someone wants to hear my mix and doesn't have surround. Well, for some reason, YouTube would never engage the 5.1 audio IF I also included a stereo track, ...no matter how I muxed it, tagged it, etc. So I don't bother with trying to include an additional stereo track.
You could issue a warning, ...it's easy enough for listeners to pause the video right before it ends. The biggest issue you might have are ADs. If a channel has ADs and one of them pops up during a 5.1 video, ...it could give someone a heart-attack!
I don't have ADs that play in the middle of any of my songs on my channel, ...only sometimes in between songs if I'm listening to a playlist; my channel is not monetized though, ...so I don't know if they would try and put an AD or two on a 5.1 video on a monetized channel or if there's a way for you to choose not to have ADs on a particular video, ...if you have that option, I would disable them on any video with 5.1 content.