Wizards of the Coast (WotC) suspended Brainstorm in Historic in an unannounced Banned & Restricted update today.
The article cited Brainstorm hampering deck diversity in the format making it less fun as the primary reason for the suspension of the card, not win rate of the main decks that play it: Izzet Phoenix and Jeskai Control.
The article states Izzet Phoenix and Jeskai Control only had 52% and 51% win rates in Best-Of-Three play, respectively. Due to the acceptable win rates, WotC expected the metagame share of the two decks would drop more than it did (only roughly 3%), leaving the decks as the most played decks in the format by a wide margin. Despite the ladder win rates, most online tournaments like the Insight Esports Historic Opens, show Izzet Phoenix having closer to a 57% win rate over the past two months.
Notably, Brainstorm is going on the suspended list instead of being banned. MTG Arena players don’t receive wild card reimbursements for suspended cards. The article states that WotC will continue to monitor the format to see if it reaches a point where it is safe to bring Brainstorm back to the format. Brainstorm is now the only card on the suspension list. No previously suspended cards are currently legal in the format other than Burning-Tree Emissary as most suspended cards normally end up banned after some amount of time or skip the suspension step altogether.
Read the original article from WotC.