WordPress mini plugin as an extension to Site Kit that ensures the inserted Google Analytics snippet anonymizes IP addresses.

google-site-kit-anonymize-ips.php

<?php
/**
 * Site Kit by Google Anonymize IPs plugin file.
 *
 * @package   Google\Site_Kit_Anomyize_IPs
 * @author    Felix Arntz, Google
 * @license   GPL-2.0-or-later
 * @copyright 2019 Google Inc.
 *
 * @wordpress-plugin
 * Plugin Name: Site Kit by Google Anonymize IPs
 * Description: Ensures the inserted Google Analytics snippet anonymizes IP addresses.
 * Plugin URI:  https://gist.github.com/felixarntz/2800ed9abfa13aabfa6e5c8ba9ec5066
 * Version:     0.1.0
 * Author:      Felix Arntz, Google
 * Author URI:  https://felix-arntz.me
 * License:     GNU General Public License v2 (or later)
 * License URI: https://www.gnu.org/licenses/gpl-2.0.html
 */

namespace Google\Site_Kit_Anomyize_IPs;

/*
 * We only need to filter Analytics options for non-AMP, since in AMP (the filter would be 'googlesitekit_amp_gtag_opt') anonymizing IPs is enabled by default
 * (see https://github.com/ampproject/amphtml/blob/master/extensions/amp-analytics/0.1/vendors/gtag.js#L50).
 */
add_filter(
	'googlesitekit_gtag_opt',
	function( array $options ) {
		// See https://developers.google.com/analytics/devguides/collection/gtagjs/ip-anonymization.
		$options['anonymize_ip'] = true;
		return $options;
	}
);