would the videos from youtube be deteriorated? Probably- depends on the quality of the upload.
Either way, you should still see the relative difference between the brightest and darkest parts of the piece, no matter what the degradation is. As well, you will still see the vectorscope leaning one way or another off of center…or right on center.
If the blacks look way to high on the scope, then they will look way too high on the version you are watching anyway…unless that is what was intended. I don’t think you will be downloading a youtube video, and suddenly see a drastically different version of the piece…depending on how your edit application interprets it.
Glenn