" One feature that isn't being immediately brought over from the legacy client are item sets. However, your existing item sets will still continue to work and our third-party API will still allow you to download custom item sets from community developers for in-game use."
This statement is in the client Beta FAQ. Located here: https://support.riotgames.com/hc/en-us/articles/230175788#h2
So it sounds like it will be implemented back in at a later time. Keyphrase = isn't being "immediately" brought over from the legacy
Also coming from The_Catopus from the Open Beta Q&A:
We're actually having to rebuild every feature for the client update, so our big challenge is deciding what order to build things in, not which things to remove. We had some data suggesting that very few players used the item set creator, so we decided to put it low on our list of priorities.
HOWEVER, we got a lot of messages from players about item sets, so we're having discussions internally about how we want to solve the problem. We'll update you when we figure it out.
In the meantime, item sets still work in games when you use the updated client, and you're free to edit them by launching the legacy client. But, like I said, we're still trying to figure out a long term solution.
Link for Q&A: https://www.reddit.com/r/leagueoflegends/comments/5di4ej/ama_were_the_league_client_update_team_ask_us/
can use Ctrl+F to search "item sets" its like the first thing to show up.
Also for the guys upset about it, open beta was optional not forced, and you can switch back any time, as stated by someone else already.