Question: when are riot skins available?
Answer: 2020 Prestige Point skins: True Damage Senna (available now) Coven Zyra (available now) Arcanist Zoe (releasing in 10.14) Yasuo. Soraka* ... 
2020 Event Prestige skins: Mecha Kingdoms Garen (event ended) Dark Star Malphite (event ended) Pulsefire Lucian (event ended) Teemo. Ezreal.