To those of you who say "I NEED CUSTOM ITEM SETS!!!!11!!1!!!"
You can still make them, there just won't be an in client interface. Sure, you'll have to actually find files and edit them manually, but if you "NEED" them, I'm sure it's worth the time.
Riot's reasoning: for the number of people who actually use item sets, the amount of time all of you spend going through making them manually < time it takes them to make an interface for it