IDK when they will add that back to the new client, but you can always use the legacy client (the old one, there's a button for it somewhere in the login screen) and make/edit your item builds there.
EDIT: Apparently, an Ask Riot post dedicated entirely to the topic of the client update was just released.
tl;dr: Use the legacy client to edit item sets for now. You should enter a game with a champion using the item set for it to be saved though (just enter a custom game then leave). Item sets doesn't seem to be an intended release function on the updated client though :/ so it'll be unavailable as soon as the legacy client becomes unavailable during March-June 2017. There'll probably be some backlash so hopefully the whole system will be back soon eventually.
EDIT2: A rioter said sometime before the end of Febraury that they'll try to bring back item sets to the new client. It'll probably take a while after the new client goes live, so we'll have a couple months of no item sets.
IDK when they will add that back to the new client, but you can always use the legacy client (the old one, there's a button for it somewhere in the login screen) and make/edit your item builds there.EDIT: Apparently, an Ask Riot post dedicated entirely to the topic of the client update was just released.
tl;dr: Use the legacy client to edit item sets for now. You should enter a game with a champion using the item set for it to be saved though (just enter a custom game then leave). Item sets doesn't seem to be an intended release function on the updated client though :/ so it'll be unavailable as soon as the legacy client becomes unavailable during March-June 2017. There'll probably be some backlash so hopefully the whole system will be back soon eventually.
EDIT2: A rioter said sometime before the end of Febraury that they'll try to bring back item sets to the new client. It'll probably take a while after the new client goes live, so we'll have a couple months of no item sets.