Detect usage of WooCommerce's legacy REST API. Logs any attempts, and displays an admin notice.

detect-legacy-wc-rest-api-usage.php

<?php
/**
 * Plugin name: Detect Legacy REST API Usage (WooCommerce)
 * Description: Attempts to detect and log usage of WooCommerce's legacy REST API.
 * Version:     2023-11-24.1
 */

function detect_and_log_wc_legacy_api_requests() {
	global $wp;

	if ( ! function_exists( 'wc_get_logger' ) ) {
		return;
	}

	$legacy_api_version = esc_html( $_GET['wc-api-version'] ?? $wp->query_vars['wc-api-version'] ?? '' );
	$legacy_api_route   = esc_html( $_GET['wc-api-route'] ?? $wp->query_vars['wc-api-route'] ?? '' );
	$user_agent         = $_SERVER['HTTP_USER_AGENT'] ?? 'unknown user agent';

	if ( empty( $legacy_api_version ) && empty( $legacy_api_route ) ) {
		return;
	}

	wc_get_logger()->info( "💡 LEGACY REST API USAGE DETECTED (version $legacy_api_version): $legacy_api_route ($user_agent)" );
	update_option( 'wc_legacy_rest_usage', array(
		'version'    => $legacy_api_version,
		'route'      => $legacy_api_route,
		'user_agent' => $_SERVER['HTTP_USER_AGENT'] ?? 'unknown',
		'datetime'   => wp_date( 'Y-m-d H:i:s' ),
	) );
}

function display_legacy_wc_api_usage_notice() {
	$legacy_usage = get_option( 'wc_legacy_rest_usage' );

	if ( ! is_array( $legacy_usage ) ) {
		return;
	}

	$defaults = array( 'version' => 'unknown', 'route' => 'unknown', 'user_agent' => 'unknown', 'datetime' => 'unknown' );
	$usage    = array_map( 'esc_html', array_merge( $defaults, $legacy_usage ) );

	echo "
		<div class='notice'>
			<p><strong>💡 LEGACY REST API USAGE DETECTED</strong> at {$usage['datetime']}</p>
			<p>
				Version <kbd>{$usage['version']}</kbd> |
				Route   <kbd>{$usage['route']}</kbd> |
				Agent   <kbd>{$usage['user_agent']}</kbd>
			</p>
		</p></div>
	";
}

add_action( 'parse_request', 'detect_and_log_wc_legacy_api_requests', -1 );
add_action( 'admin_notices', 'display_legacy_wc_api_usage_notice' );

Also might want to add a unique source prop to the log entry context. That way these log entries could be kept in a separate file from other logs.

Good suggestions ... we could definitely store a dictionary of user agents or something (probably the chief value is in—hopefully—helping to identify the sources of the requests).