Different arrangements, some allowed for under various covenants, have all been called slaves, which has led to confusion.
The bible has NEVER supported anything like the former US practice of color based slave status, or a slave status which would inhere to progeny. That was an outright blasphemy, and to be fair to the current tense state of race relations in the USA, the underlying evil (a stealing of what belongs to God) was never truly confessed by the American people.
The closest thing to the biblical practice we would know would be the former practice of “indentured servitude.” The Old Testament presented an option of permanent servitude, but the slave had to choose it and it still did not inhere to his or her progeny.
Good answer.
The main confusion is the modern attempt to reinterpret scripture in order to bring it in line with the gospel according to Saint Abraham. Onesimus was Philemon’s slave. He had run away and was with Paul. Paul didn’t tell Onesimus “good for you” and he didn’t tell Philemon that he was an evil slaveholder who needed to free Onesimus. Instead Paul told Onesimus to return to Philemon. This makes plenty of modern Christians uncomfortable so we get these convoluted attempts to explain that slavery away as “indentured servitude”.