I'm not saying that the newer APIs are backwards compatible with the older ones. I'm saying that they're not which is why they are different APIs. WinRT is a bit different since you target a Windows SDK. But, likewise newer SDKs have APIs that are not backwards compatible.
What I am saying is that the older APIs are forwards compatible with newer versions of Windows. On Windows 11, you can still run applications using those old APIs.
On Android and iOS, your old app may break when running on a newer OS version.
Microsoft doesn't have the luxury of changing the behavior of older APIs on newer versions of Windows, so they end up having to make completely new ones.
What I am saying is that the older APIs are forwards compatible with newer versions of Windows. On Windows 11, you can still run applications using those old APIs.
On Android and iOS, your old app may break when running on a newer OS version.
Microsoft doesn't have the luxury of changing the behavior of older APIs on newer versions of Windows, so they end up having to make completely new ones.