Cross platform Missing Templates after updating Visual studio 2017,i tried looking for a solution from Xamarin forums page in vain.
Yes, some templates no longer exist in VS2017, but others it's just organized a bit differently in how you get to the specific template you want.
Example: The Blank App and Blank XAML App templates no longer exist in VS2017. Instead, choose the "Cross Platform App (Xamarin.Forms or Native)" template. A wizard pops will appear allowing you to choose a native Xamarin app or a Xamarin Forms app, as well as choosing a Shared or PCL app, as well as selecting a XAML app.