How do you get the Season 2017 - Flex 5v5 Diamond?
It was a ranked reward. Riot's description in the client reads: “Earned by achieving Diamond in the 2017 ranked season in Flex 5v5.”
Is the Season 2017 - Flex 5v5 Diamond still available?
No. The Season 2017 - Flex 5v5 Diamond was a ranked reward: to get it you had to reach Diamond in Flex during Season 7 (2017). Riot does not give out past ranked rewards again, so it cannot be earned anymore.
