They've said before that, if you had to label them, the most accurate label would probably be libertarian. Which is pretty par for the course for the criticize/mock everyone and play on the shock and disgust of others for a laugh crowd, which was definitely true to a great degree in the early seasons (I say that with love for Trey and Matt).

Of course that was 20 years ago, and people change. But if I had to guess, I bet they would say that their politics have remained largely unchanged even as the show has transformed... well, transformed, a bit. I was going to say "matured" but that definitely seems wrong. We're still talking about small dick jokes and gory murder of animals which has a long proud tradition in South Park.

Anyway, any appearance to a leftward shift is largely just the rightward shift of American politics and those in power while they remain in place. No offense intended to them, but I can't imagine decades of success in media production, specifically by saying "fuck you" to everyone, has made them LESS libertarian. It's a trope for a reason.