Fatal error: ome mandatory parameters are missing ("view_mode") to generate a URL for route "flag.action_link_flag"

This was an announced API change, per https://www.drupal.org/node/3458551 → , in prior releases. Closing as "Works as designed."

I'm not sure announcing it in a changelog is enough.

First, its deprecated, not changed and 4.0 isn't out. So the expectation would be that deprecated code would continue to function issuing the warning to users.
`Not providing the "$view_mode" parameter is deprecated in flag:8.x-4.0-beta4 and will throw an error from flag:8.x-4.0. See https://www.drupal.org/node/3458551 → .`

Since it didn't, I just got a fatal error never having seen a deprecation. No chance to fix I i missed the changelog.

Second, the default code for generating links doesn't require a view mode and defaults to null.

  /**
   * Get the action link as a Link object.
   *
   * @param \Drupal\flag\FlagInterface $flag
   *   The flag entity.
   * @param \Drupal\Core\Entity\EntityInterface $entity
   *   The flaggable entity.
   * @param string|null $view_mode
   *   The flaggable entity view mode.
   *
   * @return \Drupal\Core\Link
   *   The action Link.
   */
  public function getAsLink(FlagInterface $flag, EntityInterface $entity, ?string $view_mode = NULL);

I haven't tested it yet but this would seem to trigger a fatal error when using the function as documented.

On one hand, it's a beta module so backward-incompatible changes can be expected.

On the other hand, is it really "works as expected" if it breaks other contrib modules? See 🐛 Compatibility with Flag 8.x-4.0-beta6 Active