back when .NET was first launched it was advertised as the new way of making desktop applications on Windows.
Visual C# made it very easy to design GUI interfaces.
So this "it's all for backend now" notion is surprising.
.Net is "Microsoft Java". Like Java it was designed to do everything, but as desktop development died (and mobile development was locked down by Apple and Google, limiting it to their corporate languages), it pivoted towards networked applications.
They were legally forbidden from going the Embrace-Extend-Extinguish route there, so they had to build their own version from scratch. C# exists because J++ couldn't.