Lagacy Tracked Polyfill-ish

controllers.application\.js

import Controller from '@ember/controller';
import EmberObject, { action } from '@ember/object';

function trackedData(key, initializer) {
  let values = new WeakMap();
  let hasInitializer = typeof initializer === 'function';
  
  function getter(self) {
    let value;
    if (hasInitializer && !values.has(self)) {
      value = initializer.call(self);
      values.set(self, value);
    } else {
      value = values.get(self);
    }
    return value;
  }
  
  function setter(self, value) {
    self.notifyPropertyChange(key);
    values.set(self, value);
  }
  
  return { getter, setter };
}

function legacyTracked(target, key, descriptor) {
  let { getter, setter } = trackedData(key, descriptor?.initializer);
  return {
    enumerable: true,
    configurable: true,
    get() {
      return getter(this);
    },
    set(newValue) {
      setter(this, newValue);
    },
  };
}

export default class ApplicationController extends Controller {
  @legacyTracked appName = 'FOOBAR';

  @action
  testIt() {
    this.appName = 'BARFOO';
  }
}

templates.application\.hbs

<h1>Welcome to {{this.appName}}</h1>
<br>
<br>
<button type="button" {{on "click" this.testIt}}>Click!</button>
<br>
<br>

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": "3.12.2",
    "ember-template-compiler": "3.12.2",
    "ember-testing": "3.12.2"
  },
  "addons": {
    "@glimmer/component": "1.0.0"
  }
}