By the time I gained political sentience, PC referred to perceived limits on speech and conduct that went at the expense of current or historically marginalized groups.

All these things have been derided as "politically correct" in my lifetime (and therefore a bad things): "You can't make a joke these days", i.e. you will get an earful if you make sexist, homophobic or racist jokes. "You can't give a woman a compliment these days", i.e. you can't engage in wanton sexual harassment. "Education is too politically correct these days", i.e. schools teach a history curriculum that recognizes and is critical of imperialism, racism and the like.

In my 30-odd years of life I don't recall it ever being used in the way you describe, but it could predate me.

I think your understanding is wrong although my experience with the term PC started in the early/mid 1980s and I don't know much about its use before that. What you're describing is more like "don't ask - don't tell" and "parental advisory" labels.

Typically, PC is associated with attempts to de-marginalize groups that are historically disadvantaged due to structural discrimination. To me the canonical PC is always spelling "women" as "womyn", to avoid using a term that contains the word "man", as a way to push back against perceived patriarchal naming/language systems.

You are 100% correct. The conservatives disagreeing with you (and downvoting you) are either willfully ignorant about their own history of "political correctness" or else were under a rock (or perhaps weren't alive yet) during the heyday of McCarthyism and the still-ongoing legacy thereof.

Now, when the tables are turning and companies would rather appeal to the progressive-leaning majorities than the ever-shrinking conservative minority, all of a sudden conservatives are eager to pretend that they've been the champions of the First Amendment all along (never mind that the First Amendment never applied to private platforms/businesses choosing with whom to do business).