Aha! I have located the answer…
Remove
.border-radius()and.border-*-*-radius mixins. As only Android
2.1, iOS 3.2, and older desktop browsers require a prefixed version, we’ve removed the base mixin. Since we no longer require prefixes for
independent corners, we’ve dropped those mixins as well. Mixins for a
single side, like.border-left-radius, are still available.
From TWBS Pull 6342, specifically under LESS changes.