Post-truth refers to the idea that people these days are more likely to make their minds up about particular political and social issues based on emotional response and peer pressure regardless of facts (e.g. people who just decided that Trump is a racist misogynist nazi etc because of outrage culture). It's a term that tends to get used by the same piece-of-shit media types who caused this problem in the first place. It's probably Mockingbird-related.