I don’t know the history of Reddit’s voting system, but I think controversial was a way to show posts/comments that were “unpopular” after they removed the display of both upvotes and downvotes and replaced it with percentage.
So here they could just add a sort by most downvotes, since they are displayed separately. I guess another option for sorting by most-equal upvotes vs. downvotes would be fine too, just not as simple for what would often be the same effect. Maybe sorting by most downvotes would even be preferable depending on what you’re looking for.
In other words, add a “Bottom” sort option as the simple opposite to “Top.” ( ͡° ͜ʖ ͡°)