You still need settters

controllers.application.js

import Controller from '@ember/controller';
import { computed } from '@ember/object';

export default Controller.extend({
  appName: 'Ember Twiddle',
  
  isEvenLength: computed('appName', function() {
    return this.appName.length % 2 === 0;
  }),
  
  actions: {
    withOutSetter(value) {
      this.appName = value;
    },
    
    withSetter(value) {
      this.set('appName', value);
    }
  }
});

templates.application.hbs

<h1>Welcome to {{this.appName}}</h1>

<p>There are an {{if this.isEvenLength "even" "odd"}} number of characters.</p>

<fieldset>
  <label>Without setter:</label>
  <input
    value={{this.appName}}
    oninput={{action "withOutSetter" value="target.value"}}
  >
</fieldset>

<fieldset>
  <label>With setter:</label>
  <input
    value={{this.appName}}
    oninput={{action "withSetter" value="target.value"}}
  >
</fieldset>

twiddle.json

{
  "version": "0.15.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.4.3",
    "ember-template-compiler": "3.4.3",
    "ember-testing": "3.4.3"
  },
  "addons": {
    "ember-data": "3.4.2"
  }
}