dumb

controllers.application\.js

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

export default class ApplicationController extends Controller {
  @service whatToRender;
  @tracked value = true;

  toggle = () => {
    this.value = !this.value;
  }
}

helpers.boo\.js

import Helper from '@ember/component/helper';
import { inject as service } from '@ember/service';

export default class Boo extends Helper {
  @service whatToRender;
  
  compute([yayOrNay]) {
    this.whatToRender.yayOrNay = yayOrNay;
  }
}

services.what-to-render\.js

import Service from '@ember/service';
import { tracked } from '@glimmer/tracking';

export default class WhatToRender extends Service {
  @tracked yayOrNay = false;
}

styles.app\.css

body {
  margin: 12px 16px;
  font-family: 'Helvetica Neue', Helvetica, Arial, sans-serif;
  font-size: 12pt;
}

.parent {
  display: flex;
  flex-wrap: wrap;
  max-width: 200px;
  border: 1px solid;
}

.child {
  flex: 1 0 100px;
  background: skyblue;
}

.child.nay {
  flex: 1 0 100%;
  background: lightgray;
}

templates.application\.hbs

{{boo this.value}}
<div class=parent>
	<div class="child {{unless this.whatToRender.yayOrNay "nay"}}">child 1</div>
  <div class="child {{unless this.whatToRender.yayOrNay "nay"}}">child 2</div>
</div>
<div>
	<button {{on "click" this.toggle}}>switch</button>
</div>

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"
  }
}