It might also just be a matter of growing up and suddenly realizing that everything seems so much more defined than previously thought.

Anecdotally, but I also noticed that from around 2000 to 2010, suddenly everything needed to be categorized very specifically.

But what stood out is that people would get very weirdly upset if you put something in the wrong box, and all but start a fight about it (over forums, of course.) This was especially prevalent in the electronic music scene, but happens to this day during video game genre discussions.

The "Is Zelda an RPG?" hell threads come to mind.

I think it is fine to pursue this ever increasingly detailed classification of things (not only in art or pop culture, this applies to other knowledge too, including science). The problem might come if one takes one of these descriptive, post-hoc classifications, ignores the fact that they are often diffuse and imperfect, and then tries to use it as a predictive model to explain and deal with everything, and discard any data that doesn't fit. But people like meta, so it might not even be a problem :)

My school library had maybe 3 foot shelves dedicated to scifi/fantasy.

Today I can buy the most obscure books I want on Amazon.

Maybe that is why we have more taxonomies now.

More nuance is the opposite of a problem.

Hyper-specialised labelling is the opposite of nuance.

The anti romantic movement consisting of Hawthorne and Melville comes to mind.