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).

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.