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Is there a way to make an app/game available for only xbox series s/x and not for xbox one?
Currently, I have to say that there is no such way to do it. The Device family availability options in the Store submssion contain Windows 10 Desktop, Windows 10 Mobile, Windows 10 Xbox and etc. And you could see that no further options for specific models like Xbox series s/x or Xbox One.
My suggestion is that you could submit a feature request in the Feedback Hub app. You could find the Feedback Hub app in the Start Menu.
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