Modern Second Sunrise is banned. Vintage Regrowth is unrestricted.
The deck uses Second Sunrise and Faith’s Reward to get back cards that generate mana, such as Lotus Bloom, and various artifacts that draw a card. A common choice to win the game is Pyrite Spellbomb. However, the way the player gets to the point of forcing a win might involve casting Second Sunrise so many times that the entire library is drawn, and only cards put back with Conjurer’s Bauble are left. A single turn might take fifteen minutes or more.
In a large tournament, such as a Grand Prix, when time for the round expires, players are given five additional turns to complete their game. Usually, this takes a few minutes to conclude the rest of the games. However, a player playing Eggs might have a fifteen-minute turn during the additional turns, delaying the start of the next round by ten minutes or more (beyond the next-longest match). Over the course of a day, this can mean an extra hour of waiting for everyone else in the tournament.
The DCI considered which card to ban to deal with this issue. We decided to try and do the narrowest possible ban: one that would reduce the chance of such long turns without banning a card used in other decks. That is why Second Sunrise is banned in Modern.
Regrowth was restricted in 1994. Since then, alternatives such as Yawgmoth’s Will and Snapcaster Mage have been printed, and Regrowth has seen little play. It is not clear what new decks are feasible with Regrowth unrestricted, but the DCI’s best judgment is that, after nineteen years, it is time to find out. So Regrowth is unrestricted in Vintage.