Obviously it was a good thing that it was banned, but I'm just wondering if it would technically be considered authoritarian.
As in, is any law that restricts people's freedom to do something (yes, even if it's done to also free other people from oppression as in that case, since it technically restricts the slave owner's freedom to own slaves), considered authoritarian, even if at the time that the law is passed, it's only a small section of people that are still wanting to do those things and forcibly having their legal ability to do them revoked?
Or would it only be considered authoritarian if a large part of society had their ability to do a particular thing taken away from them forcibly?
Yes.
But slavery was also authoritarian.
Any situation where there is a power imbalance that can be enforced through physical or psychological means that somebody doesn't agree with is authoritarian. Employer/employee? Authoritarian. Parent/infant? Authoritarian. Bank/bank customer? Authoritarian. Doctor/patient? Authoritarian.
Probably the only reasonable definition of authoritarian would be something like, "To be ruled/governed by an authority." I've decided that Bill over there gets to be in charge of things, they're the authority. I don't always agree with the decisions they make but they're in charge. Which seems like it would overlap a bit with the idea of democratic centralism.