Is PC just an aggregate of contemporary taboos (linguistic and/or thematic/conceptual)?
Not particularly. You can still be an asshole breaking taboos left and right AND be perfectly PC in words and behavior.
And I'm sure you know that a person can be good and not particularly PC.
Its not PC to discuss certain topics (eg. muslim attitudes towards women, or non-muslims); and its not PC to use certain terms (eg. Journalist Milo Yiannopoulos provokes by calling himself a dangerous faggot). How is this different from saying certain topics and expressions are taboo?
I don't get your train of thinking-- and I suspect it is because your use of PC is different than mine.
I don't use PC for either examples of that.
The first example is not about the topic but context.
Who's discussing such topics? Why do they discuss such topics?
Seeking internal reform is a very different thing than demonizing.
A lot of people -does- mistook the former with the latter (I live in Indonesia-- I know),
But equally a lot of people disguised the latter into the former.
The second example is consent. I can call myself a cocksucking faggot-- therefore giving myself permission to use such term about me. Until I gave you consent on saying such; you can't call me a cocksucking faggot; because that word is on itself an insult.
Are taboos an aspect of a backwards culture, or a progressive liberal culture?
Neither of them, both of them, and all of them.
The idea that a taboo (whether social, linguistic, or physical) is something specific to a certain kind of culture / group is, at best, a false dichotomy.
A group, the culture and its members has certain things that threatened their well being / survival. No matter what, no matter how diverse, no matter how singular.