How do you get the Season Year 2025 - Platinum Icon?
It was a ranked reward. Riot's description in the client reads: “This icon was earned by achieving Platinum in Season Year 2025.”
Is the Season Year 2025 - Platinum Icon still available?
No. The Season Year 2025 - Platinum Icon was a ranked reward: to get it you had to reach Platinum during the 2025 season. Riot does not give out past ranked rewards again, so it cannot be earned anymore.
