So I understand about the dead Gragas pick in Worlds, but what about Azir?

Gragas was considered gone due to his ult change, where either he cast the barrel near or far from him, the duration will be equal, making his ult easy to dodge with some champions.

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Neither Gragas nor Azir are "dead", its just other champions perform better , thats why they are picked.

Agreed, If Olaf, Elise, Reksai, lee sin or Nidalee are banned then Gragas becomes a viable option.