Teferi, Temporal Archmage
There have always been a fistful of decks revolving around spell-avalanche, High Tide close-to-combo strategies. Along Duel Commander history, most mono-blue decks have tried such a strategy, often combining the permanent suppression of a blue-based control deck and a few cards dedicated to a game-ending combo. Albeit relying on affinity key cards like The Chain Veil, Rings of Brighthearth abuses or Stasis locks, decklists showed to be very well performing, almost always transforming tournament metagame presence into top X results. Teferi, Temporal Archmage kept gaining very few key cards since then. But the pressure of such a blue deck that can answer most of the current threats combined with a fast, surprising winning combo, and a non-creature commander showed why this commander now had to be banned, despite requiring some skills to successfully resolve. Answering the combo is unfortunately usually useless, even if opponents have answers to it.