https://www.pcgamer.com/league-of-legends-reveals-loot-box-drop-rates/

Against the overarching political discourse, loot box rates continue to cause concern for players. With transparency in mind, Riot has now published the official drop rates for League of Legends' Hextech chests. 

Hextech Chests contain items from LoL's store and can be both earned for free or purchased directly. Hextech rates appear on the support section of the game's official site, contained within a list that Riot suggests will be updated "whenever we make changes to Hextech chests" including "event-specific" chests or capsules.