I suspect that people used to just have smaller, more homogeneous social groups. So saying something terrible wasn’t an issue because all of one’s associates either engaged in the same behavior or considered it too risky to speak up.

As for social justice being a recent phenomenon, it isn’t. It being on the web is, but the web is still relatively young and it finally became intertwined with our offline selves that your online behavior is just your behavior. And social consequences for that behavior can be executed online.