How do you get the 2017 Worlds Master Pickem Poro Icon?
It was a mission or event reward. Riot's description in the client reads: “Earned by scoring 34 points in Worlds Pick 'Em during 2017 Worlds.”
Is the 2017 Worlds Master Pickem Poro Icon still available?
No. Riot tied the 2017 Worlds Master Pickem Poro Icon to 2017 (mission reward), and it does not normally bring back past event rewards or bundles.
