You Can Still Use Item Sets

Read Riot's post very carefully.

They've removed the "Item Set Creator GUI". This means that any app/feature/whatever that drops item sets into your league client's directory will still work. This change literally only affects people who make custom item sets - if you use an app/program/etc to get item sets from an aggregator online, you will still be able to utilize those features (it's actually right in the FAQ where they talk about support for Championify - Championify dumps item set files directly into your League directory so anything that emulates that system will still be supported.)

Basically the reason Riot is removing the creator from the client is because most people that use Item Sets are using an aggregator program to download updated item sets on the fly from popular sites instead of making their own in the GUI. I know that I use item sets I downloaded online, but I've never once made my own, so it shouldn't affect most end users.

dunno why I'm getting downvotes for sharing true information so:

"Will the open beta be missing any features from the legacy client? Just a couple at the start. It'll take us a few patches to re-integrate spectate (for your friends and custom games) and the ability to join publicly hosted games using custom games. We're currently not planning to bring over the item set creator feature from the legacy client. Basically only an incredibly tiny percentage of players ever used the feature, and we've heard from many players that y'all don't care whether we bring item sets over so long as we continue to support 3rd party item set customization tools like Championify—and we will!"

http://na.leagueoflegends.com/en/client-update/article/4ac7a0bc-8847-40cd-8efb-aa5aa3a660f4

Emphasis mine. You can't MAKE item sets inside the game, but you can still use customs (as long as they come from an aggregator/outside app).