How do you get the 2016 World Championship?
It was sold in the League of Legends store. Riot's description in the client reads: “This icon was released in the store for purchase during the 2016 Worlds event.”
Is the 2016 World Championship still available?
Probably not. Riot tied the 2016 World Championship to 2016 (store purchase), and it does not normally bring back past event rewards or bundles.
