Activate the free shipping option only when the products in the cart all contain the same specific shipping class

woocommerce-force-free-shipping-specific-class.php

<?php 
// Replace 'shippingClass' with the specific class upon which you wish to make the free shipping option available

add_filter( 'woocommerce_shipping_free_shipping_is_available', 'free_shipping_based_on_cart_shipping_class' );

function free_shipping_based_on_cart_shipping_class( $is_available ) {
	
	$cart_items = WC()->cart->get_cart();
	$found      = false;
	
	foreach ( $cart_items as $cart_item ) {
		$product = $cart_item['data'];
		$class   = $product->get_shipping_class();
		
		if ( 'shippingClass' == $class ) {
			$found = true;
		} else {
            $other_item = true;
        }
	}
    
    if ( $other_item == true) {
        $found = false;
    }
	
	return $is_available && $found;
}