"London" Mulligan Rule Goes Official With M20
With the release of Core Set 2020, we'll be introducing a new mulligan system for all competitive Magic formats. This new mulligan was tested at Mythic Championship II in London as well as on Magic Online.
Stores may opt to enact the new rule for regular REL or lower events (including the Modern Horizons Prerelease) before the official rules change on July 5 to give players the opportunity to test out the new procedure. Competitive REL events cannot use the new rule until the official change.
Essentially, each time you take a mulligan, you draw up to seven cards, then put a number of cards from your hand equal to the number of times you have mulliganed this game on the bottom of your library in an order of your choice. Your starting hand will still be down a card for each time you mulligan, but you'll always get to select that starting hand from a choice among seven cards. Unlike the current ("Vancouver") mulligan, there's no scry after you decide your starting hand.
This rule goes into effect with the Core Set 2020 rules update. Get more information from Wizards' Ian Duke here.