I am about to build an application, an universal windows Platform app, and i can’t decide yet which way to go i have both frameworks UWP 8.1 and UWP 10, but i am not sure if i build the latest version (10) will it be backward compatible with 8.1, and if i make an 8.1 App will it run in Windows 10.
As @afree, The direct answer would be “NO”, But Microsoft recommends that instead of building a Windows 10 UWP Applications, instead build a Windows 8.1 WinRT app, that will work in all coming versions of windows including Windows 10.
As Microsoft stated: "There Won’t be any backward compatibility".
So it would be better to avoid developing for windows 10 directly, unless that is required.
Good question, but the answer still NO, As WinRT in fact is not like .NET, but it’s a whole new API, Windows OS prior Windows 10, used to have a Win32 API, but now they made a new API named WinRT. Soon or later the entire Win32 API will be deprecated.
And as you may know WinRT being an API means it’s deeply integrated into the system. it’s not just some top layer framework like .NET.