<===> input.scss
@mixin dummy($a, $b, $c, $d, $e: true) {
  content: $a $b $c $d $e;
}

.foo {
  @include dummy( ('a', 'b', 'c', 'e')..., $e: false );
}
<===> output.css
.foo {
  content: "a" "b" "c" "e" false;
}