discourse-subscriptions/assets/javascripts/discourse/routes/user-billing-index.js

import Route from "@ember/routing/route";
import { service } from "@ember/service";

export default class UserBillingIndexRoute extends Route {
  @service router;

  templateName = "user/billing/index";

  redirect() {
    this.router.transitionTo("user.billing.subscriptions.index");
  }
}