Can Windows Phone 8.1 or Windows Phone 10 applications be created in Visual Studio 2017?
Windows Phone 8.1 applications created in VS2015 are not loaded in VS2017. Instead "(incompatible)" text is displayed in Solution Explorer (I get the same behavior for Windows 8.1 applications).
(This question is not about Xamarin projects)
VS2017 doesn't support Windows Phone 8.x or Windows 8.x applications, only UWP. Windows Phone 10 application you're asking about is actually UWP app for Windows 10 Mobile, so VS2017 can do that.
VS2015 supports both types, however you won't be able to target newer versions of SDK and .NET native toolchain for UWP.