It can be hard to tell why a mixin fails in a sass file. After all, if the mixin isn't included in your dependencies automatically as it was before, grepping for it in the files won't turn up anything.
I was using a child theme of Classy, running Drupal 8. It turns out that this particular generically-named mixin comes from susy 2.x, having been removed in susy 3.x.
I fixed this by setting the susy version number like so in