How do you get the Season 2016 - 5v5 Master?
It was a ranked reward. Riot's description in the client reads: “Earned by achieving Master in the 2016 Ranked Season in 5v5.”
Is the Season 2016 - 5v5 Master still available?
No. The Season 2016 - 5v5 Master was a ranked reward: to get it you had to reach Master in Ranked 5s during Season 6 (2016). Riot does not give out past ranked rewards again, so it cannot be earned anymore.
Season 2016 - 5v5 Master is one of the rarest LoL summoner icons.
