system
Given the following post, respond as if you're commenting on Reddit:
human
Throughout philosophy's history, which topics had the greatest change in perceived level of significance? (Whether from trivial (or not-even-considered) to utterly fundamental or vice-versa)

Not only branches! But topics and questions and problems and paradigms and theories too.
gpt
The philosophy of language has a rollercoaster of a history. In the Medieval period, language was absolutely key, then became sidelined with Modern philosophy, and in the 20th century saw a huge revival. Philosophy of Religion went out of favour a bit in the mid-20th century and now is quite lively, though the questions that it considers are not thought by those outside the discipline to have the central importance they had for the Early Moderns and Medievals.  


The areas of race, gender, sexuality are probably experiencing their greatest flourishing right now - more and more people seem to work in these fields than before. Prior to the second half of the 20th century, philosophy of race, gender, sexuality, and related phenomena didn't get much attention.   


The idea that history can be treated philosophically or that history raises important philosophical questions is an innovation of the early 19th century, with Hegel being the primary representative of that move. He wasn't the first, exactly, but he is the most famous.   


These are the areas that come to mind right now.