This B&R announcement wasn't a real surprise. The Thassa's Oracle / Tainted Pact combo is really powerful and over-represented within the Historic meta. People were already shouting for Wizards to do something about this. Especially since on Arena it is such a nuisance to play against the combo. To resolve the combo as quickly as possible, it was actually faster than to just rope your opponent and wait for Arena to "handle" the rest of the turn.
Historic
MTG Arena effective date: May 20, 2021. Read the full announcement over at Wizards.
While Thassa's Oracle is a powerful card that has created combo decks across multiple formats, until recently, it wasn't a force to be reckoned with in Historic. However, with the recent printing of Tainted Pact and the fine-tuning of that archetype, we've seen the rise of the two-card combo in Historic to the point that half the field for the past League Weekend utilized the combo.
Due to the power of the two-card combination, and because we expect that Thassa's Oracle is likely to cause problems down the road as Historic continues to add new cards, Thassa's Oracle is banned in Historic.