New Twiddle

controllers.application.js

import Ember from 'ember';

export function deprecateProperty(
  object,
  deprecatedKey,
  options
) {
  function _deprecate() {
    Ember.deprecate(
      `Usage of \`${deprecatedKey}\` is deprecated. See Changelog.`,
      false,
      options
    );
  }

  Object.defineProperty(object, deprecatedKey, {
    configurable: true,
    enumerable: false,
    get() {
      _deprecate();
      return Ember.get(this, '_appName');
    },
  });
}

export default Ember.Controller.extend({
  
  _appName: 'Ember Twiddle',
  
  init() {
    this._super(...arguments);
    deprecateProperty(this, 'appName', {id: 'remove-treatment-key', until: '13.0.0' }) 
  }
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}
<br>
<br>

twiddle.json

{
  "version": "0.15.0",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.2.2",
    "ember-template-compiler": "3.2.2",
    "ember-testing": "3.2.2"
  },
  "addons": {
    "ember-data": "3.2.0"
  }
}