That's really simple in this case. The space of expressible opinions has been strictly contracted. If it had shifted in some other more complex way then I think

That's really simple in this case. The space of expressible opinions has been strictly contracted. If it had shifted in some other more complex way then I think you could have a point. In this case though the contraction was very clear, rapid, and enforced through a variety of visible and prominent site-level changes (e.g., introducing extreme quarantines or bans of many subreddits or re-engineering the frontpage to hide content [like introducing r/popular, which selectively only shows certain political content]).