[2022] Does this game need internet to play singleplayer?

Yes, it does. Singleplayer in Eco is nothing else than a multiplayer dedicated server hosted locally and the servers verify ownership of the game for joining players. Eco does not have a separate singleplayer.

As I have been stating in the past, we unfortunately do not have any plans for removing authentication of players on servers. I can totally understand your opinion on and feelings about that, given I'm a gamer as well, but this is simply a business decision.