Stop success update email for plugin/theme auto updates.

stop-auto-update-success-email.php

<?php
/**
 * Stop success email from auto-updates.
 *
 * @package StopAutoUpdateSuccessEmail
 *
 * Plugin Name:       Stop Auto-update Success Email
 * Plugin URI:        https://gist.github.com/afragen/e2f40ed2e71e590a127e8adc1db05948
 * Description:       Stop success email from auto-updates.
 * Version:           0.2.0
 * Author:            WordPress Core Contributors
 * License:           MIT
 * Requires at least: 5.2
 * Requires PHP:      5.6
 * Gist Plugin URI:   https://gist.github.com/afragen/e2f40ed2e71e590a127e8adc1db05948
 */

add_filter( 'auto_plugin_update_send_email', 'send_email_on_failure', 10, 2 );
add_filter( 'auto_theme_update_send_email', 'send_email_on_failure', 10, 2 );

/**
 * Sends update emails only when at least one plugin/theme update has failed.
 *
 * @param bool  $enabled        Whether the update email is enabled.
 * @param array $update_results An array of update results.
 * @return bool
 */
function send_email_on_failure( $enabled, $update_results ) {
	foreach ( $update_results as $update_result ) {
		if ( true !== $update_result->result ) {
			return true;
		}
	}
	return false;
}

Lets turn it around:

function plugin_update_email_on_failure( $enabled, $update_results ) {
	foreach ( $update_results as $update_result ) {
		if ( false === $update_result->result ) {
			return true;
		}
	}
    return false;
}

Tutorial added here: https://easywebdesigntutorials.com/how-to-disable-update-emails-on-success/

Should now disable success email for both plugins and themes.

Thanks Andy!