How do you get the Ann-Sivir-sary Border Icon?
It came in a bundle. Riot's description in the client reads: “An icon that entitles the border for this champion.”
Is the Ann-Sivir-sary Border Icon still available?
Probably not. Riot tied the Ann-Sivir-sary Border Icon to 2024 (bundle), and it does not normally bring back past event rewards or bundles.
