How do you get the Crabby Crab Icon?
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 2016.”
Is the Crabby Crab Icon still available?
Probably not. Riot tied the Crabby Crab Icon to 2017 (store purchase), and it does not normally bring back past event rewards or bundles.
