How do you get the Season 2019 - 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 2019.”
Is the Season 2019 - Flex Diamond Icon still available?
No. The Season 2019 - Flex Diamond Icon was a ranked reward: to get it you had to reach Diamond in Flex during Season 9 (2019). Riot does not give out past ranked rewards again, so it cannot be earned anymore.
