Anno 1800 will be discontinued on Steam after 16th April “due to a publisher decision to make the game exclusive to another PC store” states the Anno 1800 Steam page.
“The publisher has assured us that all prior sales of the game on Steam will be fulfilled on Steam, and Steam owners will be able to access the game and any future updates or DLC through Steam”
“We apologize to Steam customers that were expecting it to be available for sale after the April 16th release date”
Watch the Anno 1800 open beta trailer below:
The notice doesn’t confirm which other games store is getting the exclusive but given recent trends of other games having some sort of exclusivity with the Epic Games Store I would say this is a likely destination.