Programmatically add a footer to a view
I was able to add a Views footer with some code in a custom module.

use Drupal\views\ViewExecutable;
function YOURMODULENAME_views_pre_view(ViewExecutable $view, $display_id, array &amp;$args) {
  if ($view-&gt;id() == 'view_machine_name' &amp;&amp; $display_id === 'view_display') {
    $options = array(
      'id' =&gt; 'area_text_custom',
      'table' =&gt; 'views',
      'field' =&gt; 'area_text_custom',
      'relationship' =&gt; 'none',
      'group_type' =&gt; 'none',
      'admin_label' =&gt; '',
      'empty' =&gt; TRUE,
      'tokenize' =&gt; FALSE,
      'content' =&gt; ‘Footer content is great.’,
      'plugin_id' =&gt; 'text_custom',
    );
    $view-&gt;setHandler('view_display', 'footer', 'area_text_custom', $options);
  }
}


Reference: Drupal 8: Programmatically Adding a Views Footer