These changes will be effective from May 18th 2020.
In the weeks following the release of Ikoria: Lair of Behemoths on Magic Online, we've observed a rise in the popularity and win rate of Vintage decks using Lurrus of the Dream-Den as a companion. Because of the nature of Vintage's wide card pool and powerful restricted cards, the deck-building cost imposed by Lurrus is less restrictive relative to the payoff of having Lurrus as a companion. As a result, the win rates of several archetypes using Lurrus have surpassed 55% in Magic Online league play, and collectively decks using Lurrus are representing too large of a portion of the metagame with no indication of a shift away from this trend. Therefore, Lurrus of the Dream-Den is banned in Vintage.
As in Vintage, the wide card pool of powerful, low-mana-cost permanents in Legacy makes the power level of using Lurrus of the Dream-Den as a companion not commensurate to the deck-building cost. Several archetypes that were already strong, including Delver variants, have incorporated the use of Lurrus as a companion while necessitating relatively few deck-building changes. Collectively, Lurrus decks represent an increasingly large portion of the metagame, with several variants maintaining win rates above 55% in Magic Online league play. Matchup data indicates that metagame forces alone aren't sufficient to keep these decks in check, so we are also choosing to ban Lurrus of the Dream-Den in Legacy.
In addition, we're seeing very high win rates among decks using Zirda, the Dawnwaker as a companion in combination with Grim Monolith. While not yet widely played, Magic Online metagame data indicates that these decks would become problematic in both win rate and metagame share. Therefore, we're taking the additional step of banning Zirda, the Dawnwaker in Legacy.
For the full explanation, refer to the official announcement by WotC.