Deactivate Flat Rate Shipping if products with specific shipping classes are in the cart

gistfile1.php

/**
 * Deactivate Flat Rate Shipping if products with specific shipping 
 * classes are in the cart
 *
 * Add the shipping class slugs to the $shippingclass_array array
 */
add_filter( 'woocommerce_shipping_flat_rate_is_available', 'unset_woocommerce_shipping_methods_flat_rate', 10 ,2 );
function unset_woocommerce_shipping_methods_flat_rate ( $return, $package ) {

	// Setup an array of shipping classes that do not allow Flat Rate Shipping
	$shippingclass_array = array( 'pickup-only' );
	
	// loop through the cart checking the shipping classes
	foreach ( WC()->cart->get_cart() as $cart_item_key => $values ) {

		$shipping_class = get_the_terms( $values['product_id'], 'product_shipping_class' );

		if ( isset( $shipping_class[0]->slug ) && in_array( $shipping_class[0]->slug, $shippingclass_array ) ) {
			/**
			 * If a product in the cart has a shipping class that does not allow for Flat Rate Shipping
			 * then return false to unset the Flat Rate shipping method and break, no need to carry on
			 */
			return false;
			break;
		}

	}

	/**
	 * It we make it this far then 
	 * Flat Rate must be available, return true
	 */
	return true;

}