How do you get the Season 2015 - Solo Platinum?
It was a ranked reward. Riot's description in the client reads: “Earned by achieving Platinum in the 2015 ranked season in Solo/Duo.”
Is the Season 2015 - Solo Platinum still available?
No. The Season 2015 - Solo Platinum was a ranked reward: to get it you had to reach Platinum in Solo/Duo during Season 5 (2015). Riot does not give out past ranked rewards again, so it cannot be earned anymore.
