How do you get the Season 2018 - Solo/Duo Bronze Icon?
It was a ranked reward. Riot's description in the client reads: “Earned by achieving Bronze in the 2018 ranked season in Solo/Duo.”
Is the Season 2018 - Solo/Duo Bronze Icon still available?
No. The Season 2018 - Solo/Duo Bronze Icon was a ranked reward. To earn it, you had to reach Bronze in Solo/Duo during Season 8 (2018). Riot does not hand out past ranked rewards again, so you cannot earn this icon anymore.
