How do I add drupalSettings info from an AJAX callback
In Drupal 8, as you've discovered, drupal_add_js() has been removed.

There are two things you need to do this:

1) You cannot add JS in an ajax callback. It was a bad practices in D7, and cannot be done at all in D8. You'll need to add the JS in your form definition, not the ajax callback.

2) You add JS through a two-step process. The first is to create a new library in the *.libraries.yml file. Add a dependency to core/drupalSettings`. Then, in your form definition, you add your setting like this:

$form['some_element'] = [
  '#markup' =&gt; '', // required to render the element
  '#attached' =&gt; [
    'libraries' =&gt; [
      'some_library', // The library you created
    ],
    'drupalSettings' =&gt; [
      'modulename' =&gt; [
        'zip_code_success' =&gt; TRUE,
      ],
    ],
  ],
];