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?
Natural language is inherently imprecise. You're going to have to add a contextual definition if you want this to have a single answer.
If making someone do something is always authoritarian, abolition is authoritarian to slavers and anti-authoritarian to slaves. If implementing a law with no checks and balances is authoritarian, it was authoritarian when Louis XIV did it, but maybe not in other cases. If a policy that upholds any kind of hierarchy is authoritarian, it's always anti-authoritarian.