The nerfs are live in-game and on the league website, but the client patch notes don't mention them for some reason.
Master Yi
BASE AD 66 ⇒ 65
AD GROWTH 3 ⇒ 2.5
Q - COOLDOWN 18/17.5/17/16.5/16 ⇒ 20/19.5/19/18.5/18
W - DAMAGE REDUCTION 60/62.5/65/67.5/70%, increased to 90% for the first 0.5 seconds ⇒ 45/47.5/50/52.5/55%, increased to 90% for the first 0.5 seconds
Gwen
BASE HEALTH 660 ⇒ 620
PASSIVE - HEALING FROM DAMAGE AGAINST CHAMPIONS 60% of damage dealt ⇒ 50% of damage dealt
R - COOLDOWN 100/90/80 ⇒ 120/100/80
Sivir
Q - BASE DAMAGE 25/40/55/70/85 ⇒ 15/30/45/60/75
W - MANA COST 60 ⇒ 60/65/70/75/80
W - COOLDOWN 10 ⇒ 12