OK, I went to check the store, and dug this out for you…
Choosing which device families to support
You can uncheck the box for any Windows 10 device family if you don’t want to offer your submission to customers on that type of device. If a device family’s box is unchecked, new customers on that type of device won’t be able to acquire the app (though customers who already have the app can still use it, and will get any updates you submit).
Note There are no checkboxes for Windows 8/8.1 and Windows Phone 8.x and earlier. If your submission includes packages that can run on those OS versions, those packages will be made available to customers. To stop offering your app to customers on earlier OS versions, you’ll need to remove the corresponding packages from your submission.
In my store I can confirm there are no checkboxes for anything below Windows 10 or XBOX…
You can just create the packages and upload them, it is up to you really which it appears you will, so good luck to you. If you want to know why I say so, Amazon has dropped the Windows 8/8.1 platform entirely this year, that should be an indicator for you to not bother…