This has been suggested a few times before. The main issue is trying to categorize things. Our flair categories, for instance, are mostly geographic or topical--European History, North American History, African History, History of Religion, Military History, etc. We could do it that way, but then ancient Roman history would be mixed in with WW2.

I'm not sure there's a way that effectively categorizes posts neatly enough to be usable.

Periodization, and thus historical sub-disciplines themselves, are necessarily a compromise made to practical necessity, but we should never forget that it is not an accurate reflection of historical reality, which exists on a continuum.

In other words, history defies categorization.

I don't think it would be smart, mainly because it would only have people who read a certain section. I'd mainly be in the European history section so anything that I may tangentally learn about would be cut off. Overall, I believe it would discourage further learning.

hi.. there was more discussion on this idea a few weeks ago

[META] Introducing sub-sections based geographically or on subject area/time.

This would most certainly help the flaired users, particularly if we're short on time that day.