https://magic.wizards.com/en/articles/archive/news/may-18-2020-banned-...
Legacy
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.