Get helper

controllers.application\.js

import Controller from '@ember/controller';
import { tracked } from '@glimmer/tracking';

export default class ApplicationController extends Controller {
  @tracked fooObj;
  @tracked fooArr;
}

routes.application\.js

import Route from '@ember/routing/route';
import ArrayProxy from '@ember/array/proxy';
import ObjectProxy from '@ember/object/proxy';
const PromiseProxyArray = ArrayProxy.extend(PromiseProxyMixin);
import PromiseProxyMixin from '@ember/object/promise-proxy-mixin';
import { later } from '@ember/runloop';
import { resolve } from 'rsvp';

const PromiseProxyObject = ObjectProxy.extend(PromiseProxyMixin);

function promiseObject(maybePromise) {
  return PromiseProxyObject.create({
    promise: resolve(maybePromise)
  });
}

function promiseArray(maybePromise) {
  return PromiseProxyArray.create({
    promise: resolve(maybePromise)
  });
}

function wait(ms) {
  return new Promise((resolve) => later(resolve, ms)); 
}

async function slow(data, ms) {
  await wait(ms);
  return data;
}

export default class ApplicationRoute extends Route {
  setupController(controller, model) {
    super.setupController(...arguments); 

    controller.fooObj = promiseObject(slow({ bar: 'baz' }, 500));
    controller.fooArr = promiseArray(slow([{ bar: 'baz' }], 500));
  }
}

templates.application\.hbs

promise proxy object: {{this.fooObj}} <br>

promise proxy object resolved: {{this.fooObj.bar}} <br>

promise proxy object private content: {{this.fooObj.content.bar}} <br>

promise proxy object with get helper: {{get this.fooObj "bar"}}

<hr />

promise proxy array: {{this.fooArr}} <br>

promise proxy array resolved: {{this.fooArr.firstObject.bar}} <br>

promise proxy array private content: {{this.fooArr.content.0.bar}} <br>

promise proxy array with get helper: {{get this.fooArr "0.bar"}}  ⬅️ not working

twiddle\.json

{
  "version": "0.17.1",
  "EmberENV": {
    "FEATURES": {},
    "_TEMPLATE_ONLY_GLIMMER_COMPONENTS": false,
    "_APPLICATION_TEMPLATE_WRAPPER": true,
    "_JQUERY_INTEGRATION": true
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.5.1/jquery.js",
    "ember": "3.18.1",
    "ember-template-compiler": "3.18.1",
    "ember-testing": "3.18.1"
  },
  "addons": {
    "@glimmer/component": "1.0.0"
  }
}