I want to be able to make the implementation optional for abstract member, but it seems like it is not doable today.

abstract class Component {
    public abstract setText?(l: GetLocalization): void;
    public abstract bindInteractions?(): void;
}

I have some component classes and they MUST extend the base component class and optionally implement certain methods with a certain signature. Right now, I can only rely on good documentation.

class Post extends Component {
    public setText(l: GetLocalization): void {
        //...
    }
    // Though I don't want to implement bindInteractions method.
}

I want the power of auto-completion to also kick in.

Why is this useful?

It is useful for classes with life cycle methods.