How do you get the Season 2020 - Split 3 Icon?
It was a ranked reward. Riot's description in the client reads: “Earned by winning ranked games in Season 2020 - Split 3.”
Is the Season 2020 - Split 3 Icon still available?
No. The Season 2020 - Split 3 Icon was a ranked reward. To earn it, you had to peak the required rank during Season 10 (2020). Riot does not hand out past ranked rewards again, so you cannot earn this icon anymore.
