Originally announced for a release on Steam, Anno 1800 will be removed from Steam after its release on April 16. Due to a forming trend of games being removed from Steam and becoming Epic Store exclusives, it’s pretty safe to assume that that is what’s happening.
“Sales of Anno 1800 will be discontinued on Steam after April 16th due to a publisher decision to make the game exclusive to another PC store. 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.”
They didn’t state exactly what platform it will be an exclusive on, but after Metro: Exodus, The Division 2 and Borderlands 3 we can already guess the store it will be exclusive to. It also just so happens that if you go to the Epic Store there is no notice of removal or anything similar.
If nothing else, it's nice to hear that if you do buy it on Steam before it is removed, you will recieve continued support for it and be able to get the DLC for Anno 1800 through Steam.
Further confirming this is the fact that when The Division 2 was announced on the Epic Store, they also stated that the partnership between Ubisoft and Epic will continue with select titles which will be releasing during the year.