BundlePluginHandler should assert that plugins implement BundlePluginInterface

  public function getFieldStorageDefinitions() {
    $definitions = [];
    foreach (array_keys($this->pluginManager->getDefinitions()) as $plugin_id) {
      /** @var \Drupal\entity\BundlePlugin\BundlePluginInterface $plugin */
      $plugin = $this->pluginManager->createInstance($plugin_id);

There's no code that enforces that the plugin object actually implements BundlePluginInterface.

It would be good DX to add a check that $plugin instanceof BundlePluginInterface and throw an exception if not, so that a developer who's not added the interface gets a clear message about.

getFieldStorageDefinitions() is called only on rebuild, so it's not a performance cost.