It was mentioned back in June, most people probably just forgot about it or never read the news.
"The final Prestige Point shop has been moved back to December, when you'll have access to the full set of 2020 and 2021 prestige skins (five more than if we had run the shop next month)."
https://www.leagueoflegends.com/en-us/news/game-updates/prestige-through-end-of-2021/
In this article Riot explains how they determine who gets legendary skins : https://www.leagueoflegends.com/en-us/news/dev/ask-riot-legendary-skins/