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?
Depends on what you get to vote on, who gets to vote, if their votes count, etc.
A more democratic system could have done something like, we'll test run Brexit for a few years, make an assessment, and then allow everybody to vote again to continue Brexiting or roll it back. But that's not going to happen because ... well... representative democracy is authoritarian by design. Nobody is going to put a "Roll Back Brexit" question on a ballot who championed a pro-Brexit stance and will fight any attempt to give the people a chance to vote again (heck, they'll probably fight tooth and nail to keep any useful assessments of the effects of Brexit from being pushed into the public sphere to help voters make informed decisions as well).