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

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 if 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 #3507901: Compatibility with Flag 8.x-4.0-beta6

I'm facing the same issue even after upgrading from 9.5 to 10.5, specifically with flag 5.0, also the flag functionality not working wherever i'm using. Could any one provide the solution in my case?

Fixes missing view_mode parameter in flag.action_link_flag and flag.action_link_unflag routes.
Prevents InvalidParameterException and PHP deprecation warnings after upgrading to Flag 5.x.
Patch tested and resolves issue #3501854.

Folks, how to reproduce this? Do you use contrib or custom modules?

I can confirm this is an issue when using a contrib module like Bookmarks. This code https://git.drupalcode.org/project/bookmarks/-/blob/1.0.x/src/Plugin/Blo... triggers the error "User error: Symfony\Component\Routing\Exception\InvalidParameterException: Parameter "view_mode" for route "flag.action_link_flag" must match "[^/]++" ("" given) to generate a corresponding URL.".

I could submit a patch to add the view mode to that module, but in this use case I would just be passing "default" anyway.

I changed the default viewmode to "default" to fix old contrib modules