At least since Facebook's current community standards were written, probably earlier: We aim to prevent potential real-world harm that may be related to conten

At least since Facebook's current community standards were written, probably earlier: We aim to prevent potential real-world harm that may be related to content on Facebook. [...] We remove content, disable accounts, and work with law enforcement when we believe there is a genuine risk of physical harm or direct threats to public safety. In an effort to prevent and disrupt real-world harm, we do not allow any organizations or individuals that proclaim a violent mission or are engaged in violence, from having a presence on Facebook. This includes organizations or individuals involved in the following: - Terrorist activity - Organized hate - Mass or serial murder - Human trafficking - Organized violence or criminal activity We also remove content that expresses support or praise for groups, leaders, or individuals involved in these activities. That seems like a pretty clear statement that people who are engaged in / advocating for / recruiting for groups that engage in mass killing will be censored. It would surprise me if Facebook were the first internet platform with rules like this.