Well I would consider it as an alternative to creating a CoreModule, the documentation clearly states:
There are two ways to make a service a singleton in Angular:
Declare root for the value of the @Injectable() providedIn propertyInclude the service in the AppModule or in a module that is only imported by the AppModule
I found this here Singleton Services doc
If you app has a CoreModule of pure services you could simply get rid of it(if you don’t think is necessary of course), although I don’t recommend it, for me I consider it more mantainable to have a CoreModule because I can easily find it in the project and tells me what services are fundamental for the app and we need only one instance from them, instead of having to open a search dialog in the IDE and look for all the services that have the providedIn: 'root' setted.