This reminds me of a part of 4chan history that the old admin (moot) talked about in his final q&a, where people used to post in all sorts of different boards a

This reminds me of a part of 4chan history that the old admin (moot) talked about in his final q&a, where people used to post in all sorts of different boards all the time, but over time as the population grew, more people started staying on a single board, causing cultures to diverge, people to form their identities based on a small subset of boards, and starting cross-board flame wars and a general distaste for cross-boarders. I think these might case studies in societal behavior rather than directly being influenced by moderator action.