From Riot themselves: https://www.leagueoflegends.com/en-us/news/game-updates/prestige-through-end-of-2021/
It's going to be part of the Night & Dawn event, he's going to cost 2.000 event tokens. You can buy it later with 100 prestige points, but without the border and the icon that comes with the tokens.
You can press F12 (the default keybind) in-game, and it'll instantly save a screenshot to Riot Games\League Of Legends\Screenshots.
I'd also like to recommend ShareX over Gyazo for non-League screenshot needs in the future. Speaking from experience, much better app for the job.
Well, during the World's Event in 2019, they had two Prestige Editions in the Token Shop at the same time: Qiyana's and Riven's. I would expect it to work the same way this time, too.
In case you're curious: this second Prestige Edition will be for Volibear: https://www.leagueoflegends.com/en-us/news/game-updates/prestige-through-end-of-2021/