UX improvements to the checkout completion page

Here is how the checkout completion page looks like after the changes:

I added a checkout flow setting:

    $form['display_sidebar_checkout_complete'] = [
      '#type' => 'checkbox',
      '#title' => $this->t('Display sidebar on checkout completion'),
      '#description' => $this->t('Whether the sidebar should be shown on the checkout completion page.'),
      '#default_value' => $this->configuration['display_sidebar_checkout_complete'],
    ];

The default configuration is FALSE (again for the sake of not breaking existing sites).

I’ve tested the patch, and it works as expected.
The checkout completion page displays the updated UX elements correctly, and I didn’t encounter any issues during testing.

MR looks good to me. I tested it with both Drupal CMS and Commerce Kickstart, and it looks decent in both cases. The implementation is straightforward. The only thing that raises a yellow flag for me is the new condition for the coupon redemption form, but it feels more like fixing a bug than creating a functional regression, although I am sure this could potentially break someone's workflow.

I wonder if we should prepare a change record?

A change record might be a good idea, especially since the functionality isn't going to be turned on for existing installs.