How do I overwrite meta tags?
The invocation of the hook_html_head_alter was removed from Drupal core.
Issue - Remove invocation of hook_html_head_alter().

D8 has a new hook to achieve it : hook_page_attachments_alter().

Code from the change record :

function MYTHEME_page_attachments_alter(array &amp;$page) {
    $viewport = [
      '#type' =&gt; 'html_tag',
      '#tag' =&gt; 'meta',
      '#attributes' =&gt; [
        'name' =&gt; 'viewport',
        'content' =&gt; 'width=device-width, initial-scale=1, maximum-scale=1, user-scalable=no',
      ],
    ];
    $page['#attached']['html_head'][] = [$viewport, 'viewport'];
  }