Brush Off

In Commander, Brush Off sits at the bottom of the bounce-spell pecking order — Cyclonic Rift, Winds of Rebuke, and even Void Snare all have better cases in most shells. The cantrip keeps it from being a pure liability, and in dedicated spellslinger or spell-count decks it earns its slot by contributing to storm count or prowess triggers. Outside Commander, constructed formats have access to far more efficient interaction, so Brush Off sees essentially no competitive play in Legacy, Modern, or Pioneer despite being legal in all of them. Standard is the only context where a cantripping bounce at three mana could plausibly make a limited sideboard, and even there it's a stretch.