class amcarouselcom_contentarticle
{
	public function getList($data)
	{
		$list = array();
		return json_encode($list);
	}
}

$params = json_decode($data->params);
$categoryID = $params->category;
echo $categoryID; // result "27"

<?php
/**
* @package       Com AMCarousel
* @author        Maxim Resh
* @copyright	 Copyright © 2020 - All rights reserved.
* @license       GNU/GPLv3
**/

// No direct access
defined( '_JEXEC' ) or die;

/*
* class name preffix 'amcarousel' + 'pluginname'.
* pluginname = plugin file name
*/
class amcarouselcom_contentarticle
{
	/*
	* title, image, link, category.
	*/
	public function getList($data)
	{
		// Plugin params
		$params = json_decode($data->params);
		
		$db = JFactory::getDbo();
		$query = $db->getQuery(true);
		$query->select('a.id, a.alias, a.title, a.images, a.catid, a.language')
			->select('c.title as category_title')
			->from('#__content AS a')
			->where('a.state = "1"');
		$query->leftJoin( '#__categories as c ON c.id=a.catid' );
		// Category filter
		if ($params->category_on){
			if ($params->children_category == '0'){
				// No child categories
				$query->where('a.catid = '.$params->category);
			} else {
				// Including child categories
				$tableCategory = JTable::getInstance('category');
				$tableCategory->load( $params->category );
				$rgt = $tableCategory->rgt;
				$lft = $tableCategory->lft;
				$query->where('c.lft >= ' . (int) $lft);
				$query->where('c.rgt <= ' . (int) $rgt);
			}
		}
		// Ordering
		if ($params->ordering != 'random'){
			$query->order('a.'.$db->quoteName($params->ordering) . ' ' . $params->ordering_direction);
		} else {
			$query->order('RAND()');
		}
		// Limit
		$query->setLimit($data->count);
		
		$db->setQuery($query);
		$result = $db->loadObjectList();
		
		if (!empty($result))
		{
			// Create Title, Image, Link, Category
			$list = array();
			foreach ($result as $i=>$r)
			{
				// Title
				$list[$i]['title'] = $r->title;
				// Image src
				$im = json_decode($r->images);
				$list[$i]['image'] = $im->image_intro;
				// Item link
				JLoader::register('ContentHelperRoute', JPATH_SITE . '/components/com_content/helpers/route.php');
				$linkSlug  = $r->alias ? ($r->id . ':' . $r->alias) : $r->id;
				$list[$i]['link'] = JRoute::_(ContentHelperRoute::getArticleRoute($linkSlug, $r->catid, $r->language));
				// Category name (optional)
				$list[$i]['category'] = $r->category_title;
			}
			return json_encode($list);
		}
		return null;
	}
}