There are 3 players in an EDH match, players A, B, and C. Player A casts Modify Memory, targeting creatures controlled by B and C and it resolves, so B and C control creatures from the other player. On the next turn, B attacks and kills A.
Does the effect of Modify Memory persist now that A (the caster of the spell) has left the game?
After reading the rules, I believe that the effect does persist. The rules state that when a player leaves the game, anything controlled by that player is either returned its owner or ceases to exist. Since player A doesn't control either of the swapped creatures, there's nothing for them to return to its owner. Also, since Modify Memory has already resolved, there's no spell on the stack to remove from the game. Finally, since it's not a permanent it doesn't behave like an enchantment where its effect is removed once the controlling player leaves the game.
Modify Memory card text:
Exchange control of two target creatures controlled by different players. If you control neither creature, draw three cards.
Rule for when a player leaves the game:
800.4a When a player leaves the game, all objects (see rule 109) owned by that player leave the game and any effects which give that player control of any objects or players end. Then, if that player controlled any objects on the stack not represented by cards, those objects cease to exist. Then, if there are any objects still controlled by that player, those objects are exiled. This is not a state-based action. It happens as soon as the player leaves the game. If the player who left the game had priority at the time they left, priority passes to the next player in turn order who’s still in the game.