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