In anticipation of Runes Reforged, last preseason’s massive addition to the game, Riot decided to give everyone five extra damage to minions. This was designed to counteract the loss of runes that helped most people out in the laning phase, letting champions last hit minions more effectively.
Unfortunately, this worked a little too well. While five extra damage to minions may not sound like too much, last hitting became easier than it’s ever been and CS numbers went through the roof, especially in professional games where 300 CS in 30 minutes was the new normal.
As of Patch 8.4, Riot has decided that the change probably wasn’t in the best interest of players, and might suppress skill expression a little bit — if everyone can last hit easily it’s no longer a skill — so, they have taken away the five extra damage to minions that we started the season with.
As a whole, this should end up being a very positive change for the game, and will encourage improvement from players at all skill levels. But if you jump in a game right after you patch, and find your CS a little lower than normal, that’s why.