Composer update: Deprecation Notice: The callback \Elgg\Composer\PostUpdate::execute declared

$ composer update
Loading composer repositories with package information
Updating dependencies (including require-dev)


Deprecation Notice: The callback \Elgg\Composer\PostUpdate::execute declared at /home/projects/prj_devboxws/subdomains/dev.elgg/public_html/vendor/elgg/elgg/engine/classes/Elgg/Composer/PostUpdate.php accepts a Composer\Script\Event but post-package-update events use a Composer\Script\PackageEvent instance. Please adjust your type hint accordingly, see https://getcomposer.org/doc/articles/scripts.md#event-classes in /usr/share/php/Composer/EventDispatcher/EventDispatcher.php:303
> \Elgg\Composer\PostUpdate::execute
  - Removing phpspec/prophecy (v1.6.0)
  - Installing phpspec/prophecy (v1.6.1)
    Downloading: 100%         

Deprecation Notice: The callback \Elgg\Composer\PostUpdate::execute declared at /home/projects/prj_devboxws/subdomains/dev.elgg/public_html/vendor/elgg/elgg/engine/classes/Elgg/Composer/PostUpdate.php accepts a Composer\Script\Event but post-package-update events use a Composer\Script\PackageEvent instance. Please adjust your type hint accordingly, see https://getcomposer.org/doc/articles/scripts.md#event-classes in /usr/share/php/Composer/EventDispatcher/EventDispatcher.php:303
> \Elgg\Composer\PostUpdate::execute

Why?