How do you get the Season 2019 - Flex Platinum Icon?
It was a ranked reward. Riot's description in the client reads: “This icon was earned by finishing Platinum in Flex Queue in Season 2019.”
Is the Season 2019 - Flex Platinum Icon still available?
No. The Season 2019 - Flex Platinum Icon was a ranked reward: to get it you had to reach Platinum in Flex during Season 9 (2019). Riot does not give out past ranked rewards again, so it cannot be earned anymore.
