ASP.NET Core Angular Template: app.module.client vs. app.module.server
Let me start by prefacing that I'm not 100% on the accuracy of this statement, but since nobody else seems to have answered, I'll give it a shot.
Microsoft SPA with Angular 2 utilized Angular Universal to do the AOT rendering. It has now been upgraded to use Angular 4, which doesn't use Angular Universal. My thought is that it instead broke up the app.module.ts into a client and server file to help with AOT rendering.
The app.module.shared.ts file is actually just a global constant that is used by app.module.client.ts and app.module.server.ts. Because it all gets rendered into a couple js files during publication, it doesn't really matter that they split up the app.module file.