Fortnite's Impostors mode was indeed inspired by the popular indie game Among Us, the game’s official Twitter account has admitted.
The initial tweet brought to attention the fact Fortnite's mode now contains a new Role Bias feature which allows you to decide between being an Impostor or Agent when playing Public and Private matches. The most recent update, v18.20, also added an Impostors playlist with open Voice Chat.
In today’s tweet, Epic said the mode was “inspired by the hit Among Us game,” and further tweeted it would like to work together with InnerSloth on something “fun” sometime.