Consider adding AutoSaveData::asResponse

How would ::asResponse() then get access to all this other information?

  /**
   * Get the auto saved changes.
   */
  public function get(): CacheableJsonResponse {
    $all = $this->autoSaveManager->getAllAutoSaveList();
    $userIds = \array_column($all, 'owner');
    $cache = new CacheableMetadata();
    /** @var \Drupal\user\UserInterface[] $users */
    $users = $this->entityTypeManager->getStorage('user')->loadMultiple($userIds);
    foreach ($users as $uid => $user) {
      $access = $user->access('view label', return_as_object: TRUE);
      $cache->addCacheableDependency($user);
      $cache->addCacheableDependency($access);
      if (!$access->isAllowed()) {
        unset($users[$uid]);
      }
    }
    // User display names depend on configuration.
    $cache->addCacheableDependency($this->configFactory->get('user.settings'));

    // Remove 'data' key because this will reduce the amount of data sent to the
    // client and back to the server.
    $all = \array_map(fn(array $item) => \array_diff_key($item, ['data' => '']), $all);

    $withUserDetails = \array_map(fn(array $item) => [
      // @phpstan-ignore-next-line
      'owner' => \array_key_exists($item['owner'], $users) ? [
        'name' => $users[$item['owner']]->getDisplayName(),
        'avatar' => $this->buildAvatarUrl($users[$item['owner']]),
        'uri' => $users[$item['owner']]->toUrl()->toString(),
        'id' => $item['owner'],
      ] : [
        'name' => new TranslatableMarkup('User @uid', ['@uid' => $item['owner']]),
        'avatar' => NULL,
        'uri' => NULL,
        'id' => $item['owner'],
      ],
    ] + $item, $all);
    return (new CacheableJsonResponse($withUserDetails))->addCacheableDependency($cache->addCacheTags([AutoSaveManager::CACHE_TAG]));
  }

I think we can close this now we have ::isEmpty

My main gripe was that we were checking $autosave->data === NULL outside of the AutoSave Data class which was leaking an implementation detail.

The special case of NULL is unfortunate but I can live with it if _only_ the class itself is aware of what it means.

With ::isEmpty that is now the case (and I fixed the other is null check when we added isEmpty).

So yep, closed duplicate on 📌 Code Components should render with their auto-saved state(if any) when rendered in the XB UI Active which fixed that