The Commonwealth never outlawed slavery until after the Civil War. There were court cases that made it difficult but slavery was illegal. Slave owners started calling enslaved blacks indentured servants but there was no provision to free them. There were even slaves in Lexington - the Cradle of Liberty.
There were court cases that made it illegal. Just because the Massachusetts legislature never passed a law outlawing it is irrelevant. I'm not aware of a single Southern state that passed a law outlawing slavery either, but the 13th Amendment kind of made such laws irrelevant.
Before the Revolutionary War, more than half of immigrants here came over as indentured servants.
That was a legal status whereby young people sold their services for a number of years in exchange for transportation to America.
After 1800 various new laws made indentured service more difficult to enforce, and therefore less frequently used.
But by 1860 the practice was still legal, and there is no reason to assume that the term "indentured servant" in, say Massachusetts, must necessarily have really meant black slaves.
The United States formally outlawed indentured servitude in the year 2000 as part of the Victims of Trafficking and Violence Protection Act (VTVPA).