This code allows you to make some WooCommerce Checkout Fields optional.

orbisius_quick_fix_wc_ext_opt_co_fields.php

<?php
/*
This code allows you to make some WooCommerce Checkout Fields optional.
Blog post http://orbisius.com/link/3710
Author: Svetoslav Marinov (Slavi)
Author URI: http://orbisius.com
*/

/*
Installation
1. Paste the code below in your functions.php OR create an mu-plugin in wp-content/mu-plugins/orbisius_quick_fix_wc_ext_opt_co_fields.php
2. Define which fields you want optional by putting this in your wp-config.php
You can see the fields on the blog post about this fix at http://orbisius.com/link/3710

define('ORBISIUS_QUICK_FIX_WOO_OPTIONAL_CHECKOUT_FIELDS', 'first_name,last_name' );

3. If you want a customizaiton ($) contact me.
*/

add_filter( 'woocommerce_checkout_fields', 'orbisius_quick_fix_wc_ext_opt_co_fields_make_optional_fields', 25);
add_filter( 'woocommerce_billing_fields', 'orbisius_quick_fix_wc_ext_opt_co_fields_make_optional_fields', 25);
add_filter( 'woocommerce_default_address_fields', 'orbisius_quick_fix_wc_ext_opt_co_fields_make_optional_fields', 25);

/**
 * This function does the heavy lifting to make fields optional.
 * @param array $fields
 * @return array
 * @copyright (c) 2017 Slavi Marinov, http://orbisius.com
 */
function orbisius_quick_fix_wc_ext_opt_co_fields_make_optional_fields($fields) {
    static $optional_fields = null;

    // Don't know what to process so let's get out of here.
    if (!defined('ORBISIUS_QUICK_FIX_WOO_OPTIONAL_CHECKOUT_FIELDS')) {
        return $fields;
    }
    
    if (is_null($optional_fields)) {
        $optional_fields = preg_split('#\s*[\,\;\|\r\n]+\s*#si', ORBISIUS_QUICK_FIX_WOO_OPTIONAL_CHECKOUT_FIELDS );
        $optional_fields = array_unique($optional_fields);
        $optional_fields = array_filter($optional_fields);
    }
    
    // Nothing to process so let's get out of here.
    if (empty($optional_fields)) {
        return $fields;
    }
    
    foreach ($optional_fields as $field_key) {
        if (isset($fields[$field_key])) {
            $fields[$field_key]['required'] = false;
        }
        
        if (isset($fields['shipping'][$field_key])) {
            $fields[$field_key]['shipping']['required'] = false;
        }
 
        if (isset($fields['billing'][$field_key])) {
            $fields[$field_key]['billing']['required'] = false;
        }
        
        // do we have fields that don't have a prefix?
        $no_prefix_key = $field_key;
        $no_prefix_key = preg_replace( '#(billing|shipping)_#si', '', $no_prefix_key);
        
        if (isset($fields[$no_prefix_key])) {
            $fields[$no_prefix_key]['required'] = false;
        }
        
        if (isset($fields['shipping'][$no_prefix_key])) {
            $fields[$no_prefix_key]['shipping']['required'] = false;
        }
        
        if (isset($fields['billing'][$no_prefix_key])) {
            $fields[$no_prefix_key]['billing']['required'] = false;
        }
    }

    return $fields;
}