I have Irish family members who were sold into slavery. While doing genealogy research I found numerous records of more than a dozen of my Irish ancestors who were enslaved, both as children and adults. Thus far, I have found death records, but no evidence whatsoever that any were ever freed. My most recent ancestor who came here as an indentured servant was my grandmother who arrived in 1913. She worked as a maid for 6 years, receiving only room and board as compensation. Her labor was to repay her sponsor. I actually knew her. How many descendants of African slaves can honestly say they knew a relative who was an indentured servant?
Indentured servitude is still practiced today, mostly in the middle east, and it was certainly an informal system well into the 20th Century. It was usually tied to the cost of passage. Many people signed agreements to work in exchange for payment of their passage to the New World. In the colonial era that was formalized in records of indenture, many of which still exist in the records. The practice was also certainly abused through failure to release people from their indenture. The same thing happened in the mines, railways, and textile mills (”I owe my soul to the company store”).
But, that was not the institution of slavery. That practice was limited by Western nations to non-christian populations. In the Islamic world, muslims could not be sold into slavery but Christians were fair game.