Anno 1800 was available for purchase on Steam prior to release but at some point they made a deal with Epic to sell it there for a year. Then it was removed from Steam. If you already bought it you could use it on Steam but everyone else had to wait. You could also directly buy it from Ubisoft’s own store Uplay so in the most strict sense it was not an exclusive contract but pretty damn close. Also it wasn’t a secret. The company talked about it. They had to, because it was literally available for pre purchase on Steam and then suddenly wasn’t.