How do you get the 2019 Worlds Team Flash Icon?
It came in a bundle. Riot's description in the client reads: “Part of the team supporter bundle for Worlds 2019”
Is the 2019 Worlds Team Flash Icon still available?
Probably not. Riot tied the 2019 Worlds Team Flash Icon to 2019 (bundle), and it does not normally bring back past event rewards or bundles.
