E-C getters work

controllers.application\.js

import Controller from '@ember/controller';
import { task, timeout } from 'ember-concurrency';

export default class ApplicationController extends Controller {
  @(task(function * () {
    yield timeout(1000);
  }))
  fooTask;
  
  get isLoading() {
    return this.fooTask.isRunning;
  }
}

templates.application\.hbs

<p>
  {{#if this.isLoading}}
    Loading
  {{else}}
    Idle
  {{/if}}
</p>
<p>
  <button
    type="button"
    {{on "click" (perform this.fooTask)}}
  >
    Go
  </button>
</p>

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",
    "ember-concurrency": "2.0.2"
  }
}