/**
 * Implements hook_update_projects_alter().
 */
function {{ machine_name }}_update_projects_alter(&$projects) {
  // Hide a site-specific module from the list.
  unset($projects['site_specific_module']);

  // Add a disabled module to the list.
  // The key for the array should be the machine-readable project "short name".
  $projects['disabled_project_name'] = [
    // Machine-readable project short name (same as the array key above).
    'name' => 'disabled_project_name',
    // Array of values from the main .info.yml file for this project.
    'info' => [
      'name' => 'Some disabled module',
      'description' => 'A module not enabled on the site that you want to see in the available updates report.',
      'version' => '8.x-1.0',
      'core' => '8.x',
      // The maximum file change time (the "ctime" returned by the filectime()
      // PHP method) for all of the .info.yml files included in this project.
      '_info_file_ctime' => 1243888165,
    ],
    // The date stamp when the project was released, if known. If the disabled
    // project was an officially packaged release from drupal.org, this will
    // be included in the .info.yml file as the 'datestamp' field. This only
    // really matters for development snapshot releases that are regenerated,
    // so it can be left undefined or set to 0 in most cases.
    'datestamp' => 1243888185,
    // Any modules (or themes) included in this project. Keyed by machine-
    // readable "short name", value is the human-readable project name printed
    // in the UI.
    'includes' => [
      'disabled_project' => 'Disabled module',
      'disabled_project_helper' => 'Disabled module helper module',
      'disabled_project_foo' => 'Disabled module foo add-on module',
    ],
    // Does this project contain a 'module', 'theme', 'disabled-module', or
    // 'disabled-theme'?
    'project_type' => 'disabled-module',
  ];
}