magento-test/app/code/Magento/Elasticsearch7/etc/search_engine.xml

<?xml version="1.0"?>
<!--
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
-->
<engines xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Search/etc/search_engine.xsd">
    <engine name="elasticsearch7">
        <feature name="synonyms" support="true" />
    </engine>
</engines>