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YouTube tests removing viewer counts — here’s what we know
(www.tomsguide.com)
This is a most excellent place for technology news and articles.
What type of reaction content? If it wasn't using the minimum amount of copyrighted material needed to comment or being transformative, and was distributing the majority of a work, then at any point a DMCA will nuke em. Google might not think it's worth the risk hosting that reaction content forever.
Basically they go on reddit, and will browse subs like "instant regret" or "maybe maybe maybe" or "yes yes no". Basically just subs where people post videos of themselves, doing whatever. Essentially it's just Americas Funniest Home Videos, but with their commentary using reddit as the source of the videos.
So, I'm not sure anything is actually "copyrighted".
I doubt that would be the reason for their ban. If it is a creative work then in some countries it's automatically copyrighted, but it's not like most would go out of their way to stop it (unlike with like people reacting to other YouTube videos, films, anime, etc).