How do you get the 2018 Worlds MAD Team Icon?
It came in a bundle. Riot's description in the client reads: “This icon was part of a bundle exclusive for 2018 Worlds event”
Is the 2018 Worlds MAD Team Icon still available?
Probably not. Riot tied the 2018 Worlds MAD Team Icon to 2018 (bundle), and it does not normally bring back past event rewards or bundles.
