Mana Drain

• WWizards of the Coast2020-11-10
  Official Ruling
  Mana Drain's delayed triggered ability will usually trigger at the beginning of your precombat main phase. However, if you cast Mana Drain during your precombat main phase or during your combat phase, its delayed triggered ability will trigger at the beginning of that turn's postcombat main phase.
• WWizards of the Coast2020-11-10
  Official Ruling
  If the target spell is an illegal target by the time Mana Drain tries to resolve, Mana Drain doesn't resolve. You don't add mana at the beginning of your next main phase. If the target is legal but not countered (most likely because an effect says that the spell can't be countered), you do add mana.

Rulings via Scryfall

• WO
  Wizards of the Coast2025-11-10

  Mana Drain was banned in brawl on November 10, 2025. Mana Drain has been controversial since its debut. Due to its generic nature as a counterspell, it is an auto-include in any blue deck and is homogenizing. The experience of having to play around or lose to an early Mana Drain against any blue deck is one that we're happy to remove from Brawl for now. Source: https://magic.wizards.com/en/news/announcements/banned-and-restricted-november-10-2025
• WO
  Wizards of the Coast2024-04-07

  Mana Drain was banned in historic on April 7, 2024. Another defining characteristic of Historic is its overall card power level, particularly now that Timeless exists as the highest-powered format on Arena. The Breaking News Bonus sheet brings some clear power outliers, so Mana Drain and Reanimate are banned in Historic. Source: https://magic.wizards.com/en/news/mtg-arena/mtg-arena-announcements-april-8-2024