How do you get the Season 2020 - Flex Diamond Icon?
It was a ranked reward. Riot's description in the client reads: “This icon was earned by finishing Diamond in Flex Queue in Season 2020”
Is the Season 2020 - Flex Diamond Icon still available?
No. The Season 2020 - Flex Diamond Icon was a ranked reward: to get it you had to reach Diamond in Flex during Season 10 (2020). Riot does not give out past ranked rewards again, so it cannot be earned anymore.
