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 earn it, you had to reach Diamond in Flex during Season 10 (2020). Riot does not hand out past ranked rewards again, so you cannot earn this icon anymore.
