Quick Start: Missing PHP extensions shown as "Array"

Is install_display_requirements actually broken or is this more of a CLI display-time issue?

This is a bug in install_display_requirements(), or at least an incorrect assumption.

system_requirements() builds the description as a render array as follows:

  $requirements['php_extensions'] = [
    'title' => t('PHP extensions'),
  ];
...
    $description = t('Drupal requires you to enable the PHP extensions in the following list (see the <a href=":system_requirements">system requirements page</a> for more information):', [
      ':system_requirements' => 'https://www.drupal.org/docs/system-requirements',
    ]);

    // We use twig inline_template to avoid twig's autoescape.
    $description = [
      '#type' => 'inline_template',
      '#template' => '{{ description }}{{ missing_extensions }}',
      '#context' => [
        'description' => $description,
        'missing_extensions' => [
          '#theme' => 'item_list',
          '#items' => $missing_extensions,
        ],
      ],
    ];

    $requirements['php_extensions']['value'] = t('Disabled');
    $requirements['php_extensions']['severity'] = REQUIREMENT_ERROR;
    $requirements['php_extensions']['description'] = $description;

But then in install_display_requirements() it is assumed that the description is a string, and it is concatenated directly:

          $failures[] = $requirement['title'] . ': ' . $requirement['value'] . "\n\n" . $requirement['description'];

The hook_requirements() doc does not state the format of the description key, it only says:

 *   - description: The description of the requirement/status.

Duplicate of #2833864: Unmet installation requirements may contain render elements