With the newly announced unification of the Pauper format, Wizards of the Coast is preemptively banning three cards that would have been added to the format: High Tide, Hymn to Tourach, and Sinkhole.
400+ new cards will be added to Pauper when cards that were printed at Common in paper, but not on Magic Online, become legal in Pauper on July 2, 2019. These cards include both High Tide and Merchant Scroll, which together form the core of a powerful combo that would have become legal. Wizards said that that combo would have been too strong for the format and that they believed that they had to ban at least one of its pieces. In their opinion, High Tide was the more narrow and difficult-to-interact-with spell, while Merchant Scroll can be played in decks other than combo-focused lists, so they decided to ban High Tide.
A powerful combo that would have been legal in Pauper
Sinkhole and Hymn to Tourach would also have been legal in newly-unified Pauper format. However, Wizards said flatly that they are both too powerful for the format and extremely unfun to play against, so they preemptively banned both cards.
Both too powerful AND unfun.
