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

Vintage

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.

We recognize that it's a rare occurrence to ban a card for balance reasons in Vintage rather than restricting it, but this is a unique case where restricting Lurrus wouldn't affect its usage as a companion, which is the primary motivation for making this change.