How do you get the Season 2020 - Split 1 Icon?
It was a ranked reward. Riot's description in the client reads: “Earned by winning ranked games in Season 2020 - Split 1.”
Is the Season 2020 - Split 1 Icon still available?
No. The Season 2020 - Split 1 Icon was a ranked reward: to get it you had to peak the required rank during Season 10 (2020). Riot does not give out past ranked rewards again, so it cannot be earned anymore.
