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