Conditionally show/hide WooCommerce checkout fields - http://calebburks.com/conditionally-showhide-woocommerce-checkout-fields

wc-ninja-conditional-checkout-fields.php

<?php
/**
 * Plugin Name: Conditional WooCommerce Checkout Fields
 * Description: Adds the abilitiy to conditionally show / hide checkout fields.
 * Version: 1.0
 * Author: Caleb Burks
 * Author URI: http://calebburks.com
 */


/**
 * Check if a specific product ID is in the cart
 */
function wc_ninja_product_is_in_the_cart( $ids ) {
	// Products currently in the cart
	$cart_ids = array();

	// Find each product in the cart and add it to the $cart_ids array
	foreach( WC()->cart->get_cart() as $cart_item_key => $values ) {
		$cart_product = $values['data'];
		$cart_ids[]   = $cart_product->id;
	}

	// If one of the special products are in the cart, return true.
	if ( ! empty( array_intersect( $ids, $cart_ids ) ) ) {
		return true;
	} else {
		return false;
	}
}

/**
 * Check if a specific product category is in the cart
 */
function wc_ninja_category_is_in_the_cart( $categories ) {
	// Products currently in the cart
	$cart_ids = array();

	// Categories currently in the cart
	$cart_categories = array();

	// Find each product in the cart and add it to the $cart_ids array
	foreach( WC()->cart->get_cart() as $cart_item_key => $values ) {
		$cart_product = $values['data'];
		$cart_ids[]   = $cart_product->id;
	}

	// Connect the products in the cart w/ their categories
	foreach( $cart_ids as $id ) {
		$products_categories = get_the_terms( $id, 'product_cat' );

		// Loop through each product category and add it to our $cart_categories array
		foreach ( $products_categories as $products_category ) {
			$cart_categories[] = $products_category->slug;
		}
	}

	// If one of the special categories are in the cart, return true.
	if ( ! empty( array_intersect( $categories, $cart_categories ) ) ) {
		return true;
	} else {
		return false;
	}
}

/**
 * Check if a specific coupon was used
 */
function wc_ninja_coupon_was_used( $coupons ) {
	// Coupons Used
	$coupons_used = WC()->cart->applied_coupons;

	// If ones of our special coupons were used, return true.
	if ( in_array( $coupons, $coupons_used ) ) {
		return true;
	} else {
		return false;
	}
}

/**
 * Conditionally remove checkout fields
 */
function wc_ninja_remove_checkout_field( $fields ) {
	$product_ids = array( '45', '70', '75' );
	$categories  = array( 'clothing', 'posters' );
	$coupons     = array( 'flat_discout', 'free_shipping' );

	// If a special coupon was not used, hide the Company Field
	if ( ! wc_ninja_coupon_was_used( $coupons ) ) {
		unset( $fields['billing']['billing_company'] );
	}

	// If a special category is in the cart, hide the First Name Field
	if ( wc_ninja_category_is_in_the_cart( $categories ) ) {
		unset( $fields['billing']['billing_first_name'] );
	}

	// If a special product is not in the cart, hide the Phone Number Field
	if ( ! wc_ninja_product_is_in_the_cart( $product_ids ) ) {
		unset( $fields['billing']['billing_phone'] );
	}

	return $fields;
}
add_filter( 'woocommerce_checkout_fields' , 'wc_ninja_remove_checkout_field' );

Hi, thank you for this one. Also, for anyone who will be using this, take note that

$cart_ids[] = $cart_product->id;

the 'id' will give an error to the newer woocommerce version. What I did is,

$cart_ids[] = $cart_product->get_id();

using get_id() instead of id.