Political correctness.

One strong implication of this discussion is that bickering over political correctness isn't a social media phenomenon at all.

I would say that 9/11 interrupted the first wave of political correctness for about 5-7 years. The 1990s were full of some oddly different anxieties from our own, but also some that were quite similar. And left and right occupied similar positions about these anxieties. Perhaps there was less penetration of ideas from the academy into mainstream media, but that's harder to say.

During the immediate post-9/11 era PC anxieties were set aside, because it really felt like we were at war. This is a thing that many young people alive today do not remember or never experienced. Being alive in 2002 was absolutely terrifying, and worrying about gendered pronouns or table displays featuring cotton seemed... impolite, almost. And certainly trivial.

The takeaway here is that NO, political correctness is not a social media phenomenon. It was already well underway in the 90s; it was briefly interrupted; and now it's back. Structural causes, if they exist, began probably sometime in the late 1980s to early 1990s. We should look there and give the question further thought. I don't have a ready answer.