Which scientific discipline studies topics?
Sociology could be one consideration. The word “topic” has the ability to shift, what could be considered a “theme” is dynamic, and evolves with societies definition of what it means. Philosophy could also be a major player because you’re concerned with the question of what it means to be a topic, and that sounds like the realm of philosophical thought. To me it sounds more like a question best suited for the social sciences as opposed to the hard sciences.