At that point I remembered to switch to Steam, in order to remove Going Medieval from my wishlist there. I know me; I would be perfectly able to buy Going Medieval again in a few months on Steam at some sale, not remembering I already have it on Epic Games. So I started thinking, maybe there is a way to add the game bought on Epic Games to my Steam library? Well, there is and there isn't. You can add any game to your Steam library with the "Add a non-Steam game to my library" function. But that will only enable you to start the game from your Steam library. It will *not* update your wishlist, or show that you already own the game. It would be possible with me to end up with two copies of Going Medieval in my Steam library. This is not the function I was looking for.

But, impressively, there is another function in the Steam store: If you go to the Steam store page of the game, next to the wishlist button and the follow button, there is an "ignore" button. And if you click on the little down arrow next to that ignore button, there is actually an option "Played on another platform". That hides the game from the Steam store, but still can use it to create recommendations. Great! It is quality of life features like that which make Steam my primary PC games platform, and everything else secondary. You could only beat that with a "import games library from another platform" button, and that is probably beyond the limit of cooperation between those platforms.