How do you get the Season 2019 - TT Master Icon?
It was a ranked reward. Riot's description in the client reads: “This icon was earned by finishing Master in Twisted Treeline in Season 2019.”
Is the Season 2019 - TT Master Icon still available?
No. The Season 2019 - TT Master Icon was a ranked reward: to get it you had to reach Master in Twisted Treeline 3v3 during Season 9 (2019). Riot does not give out past ranked rewards again, so it cannot be earned anymore.
Season 2019 - TT Master Icon is one of the rarest LoL summoner icons.
