For a given component all its members (methods, properties) accessed by its template must be public in the AOT compilation scenario. This is due to the fact that a template is turned into a TS class. A generated class and a component are 2 separate classes now and you can’t access private members cross-class.
In short: you can’t access private members in your templates if you want to use ahead-of-time compilation.
For better explaination https://github.com/angular/angular/issues/11422