->-bleeped-<- on a voluntary basis has been with us since time immortal, we all have illustrious ancestors.
Genital mutilation has also been with us for an equal amount of time. Until very recently in non-Christian societies, soldiers captured in battle were enslaved, and castrated to reduce the urge to revolt or run away. Full genital removal was usually reserved for those who were destined to be in touch with the women of the slave household. In Christian societies though the Renaissance, full genital removal was reserved for those caught in, or suspected to have been, engaging in acts of sodomy.
In some royal societies where the big honcho had hundreds of wives and thousands of of children, the glans of the penis was removed in some of the males, to keep them from reproducing but not to reduce their masculine vigor. This helped keep royal families small, and supposedly less involved in dynastic quarrels. The world would be a better place if the Saudi royal family had adopted this practice.
The Venetian castrati were a special case. It was semi-voluntary, in the sense that those castrated before puberty received a good life afterwards. Those who fell into the category of being eligible for castrato status all came from orphanages. Each orphanage had a boys choir, and the opera companies would come to performances and select those with the best voices. It was semi-voluntary because everyone knew that you could easily escape by not singing well. And as the fate of those orphans not so chosen was to end up rowing in the Venetian galleys, there was some incentive to sing well and accept the consequences. Some of these might well have seen themselves as trans, but as the survival rate for those being castrated was only about 80 percent, there was some risk involved.
On the female side, the girls' orphanages would have dances so the squires of Venice could select their concubines. Similar to the Octoroon Balls in New Orleans. Those not chosen for their beauty had little option after puberty than to become prostitutes, following in the footsteps of their purported mothers.
In the Islamic world, removal of the sensate parts of the female genitals was intended to assure the future husband that his wife would not be tempted to stray regardless of his performance, or lack thereof, in the conjugal bed. They were made into baby-making machines, nothing else.
Slavery in the Americas did not involve genital mutilation. In the ancient world, slavery was not inherited, the offspring of a slave was free, and it was not a good idea to have a lot of free citizens around who might have a chip on their shoulder about the way their parents had been treated. In the Americas the children of slaves were also enslaved, so it was in the interest of the owners not to mess with genitals, and have their slaves reproduce at the maximum rate. Interest on the slave owners investment, so the speak.
People have been doing things to other peoples genitals for a very long time. For the sake of vocabulary, however, I would only put the volunteers in the category of trans. And there were more than enough of those.