Displays a backtrace for deprecation notices.

00-deprecation-backtrace.php

<?php
/**
 * Plugin Name: Deprecation Backtrace
 * Description: Shows the line number and file where a deprecation notice occurs.
 * Version: 1.0.0
 * Author: WooCommerce
 */

// https://westi.wordpress.com/2012/01/28/tracing-things-back-to-where-they-came-from/
// https://westi.wordpress.com/2012/01/28/tracing-things-back-to-where-they-came-from/#comment-14182
function deprecation_notice_backtrace() {
	error_log( wp_debug_backtrace_summary() );
}

add_action( 'doing_it_wrong_run', 'deprecation_notice_backtrace' );
add_action( 'deprecated_argument_run', 'deprecation_notice_backtrace' );
add_action( 'deprecated_function_run', 'deprecation_notice_backtrace' );