It’s a broad question, but my understanding is that eunuchs existed across large swaths of the world and for a very long time, and I’m not even sure where one would start researching them.
What were the contexts surrounding the mentions of eunuchs in the bible? written by u/caffarelli talks about eunuchs in the bible; links to related idea by u/yodatsracist; and has a top level comment linking to previous threads by u/caffarelli
Despite the title Did the bizarre practice of turning men into eunuchs (and then giving these men substantial responsibilities) emerge... written by u/caffarelli discusses the idea of gender and roles in some the many replies to follow ups
Why were eunuchs created by so many cultures in the past? What exactly were these cultures trying to accomplish by removing the testicles? written by u/caffarelli and u/XenophonTheAthenian
What sort of roles did eunuchs fulfill in the Ottoman Empire? written by u/laklota101 and u/caffarelli (noticing a pattern yet?)