What exactly do you mean by Christendom forbidding slavery? Do you mean as in a bible verse, a decree from the Pope, or what?

I'm asking because there are arguably pretty late instances of individual Christians practicing slavery (in the United States for example), and I don't know if they considered it to be forbidden at the time.

I can only speak of Catholicism and slavery.

The first Pope to publicly condemn slavery was Benedict XIV in 1741. That being said, the church did not come down hard on slavery until Pius VII in the aftermath of the Napoleonic Wars.

Well into the 16th century the Church publicly supported slavery. Pope Paul III authorized the purchase of Muslim slaves in 1548.

From a religious standpoint, 1839 is generally considered the year the slavery was "banned" by the Catholic Church. Gregory XVI's In Supremo Apostolatus was a papal bull officially condemning slavery. It was followed by the Church working to end colonial slavery, most famously in Brazil.