How do you get the Season 2016 - Solo Diamond?
It was a ranked reward. Riot's description in the client reads: “Earned by achieving Diamond in the 2016 Ranked Season in Solo.”
Is the Season 2016 - Solo Diamond still available?
No. The Season 2016 - Solo Diamond was a ranked reward: to get it you had to reach Diamond in Solo/Duo during Season 6 (2016). Riot does not give out past ranked rewards again, so it cannot be earned anymore.
