Seems like a mistake to not ban Mana Drain. Going to lead to a ton of non-games when the opponent just counters your 3 drop and suddenly has 6+ mana on like turn 3. Way too swingy for a 1v1 format that isn't Timeless, IMO.
No prebans definitely doesn't mean we won't be making any bans. Mana Drain is a heck of a card, and we're planning to keep a close watch on it wherever it's legal. We may well end up needing to take action, but we'll make that call based on what players show us