add-rss-feed-summary-params.php

<?php
/**
 * Add RSS Feed Summary Params
 *
 * Source: https://wordpress.stackexchange.com/a/195197
 */
add_action('rss_tag_pre', function ($tag) {
	$summary = filter_input(INPUT_GET, 'summary', FILTER_SANITIZE_STRING);
	if ('yes' === $summary)
		add_filter('option_rss_use_excerpt', '__return_true');
	elseif ('no' === $summary)
		add_filter('option_rss_use_excerpt', '__return_false');
});