Not tested but it should probably look something like this.
class ParentPage extends Page
{
/**
* Creates and stores a new page
*
* @param array $props
* @return Page
*/
public static function create(array $props): Page
{
$props['slug'] = $props['content']['title'] . '-' . date('Y');
return parent::create($props);
}
}
Note:
Since Kirby 3.4 you need to overwrite the create method on the actual page model that is going to be created. Not the parent model!