Disable WooCommerce Free Shipping based on the assigned product category (in this case category with ID #112)

no-free-shipping-product-categories.php

<?php

/**
 * Disable free shipping for select WooCommerce product categories
 * - In this case, free shipping is disabled when a product has the category with ID 112 assigned to it
 * @param bool $is_available
 */
function disable_free_shipping_for_certain_product_categories( $is_available ) {
	global $woocommerce;
	// set the product ids that are ineligible
	$ineligble_categories = array('112');
	// get cart contents
	$cart_items = $woocommerce->cart->get_cart();
	// loop through the items looking for one in the ineligible array
	foreach ( $cart_items as $key => $item ) {
		$product_categories = wp_get_object_terms( $item['product_id'], 'product_cat', array( 'fields' => 'ids' ) );
		if( $product_categories ) {
			foreach( $product_categories as $category_id ) {
				// 112 -> Gynamistic Mats
				if( in_array( $category_id, $ineligble_categories ) ) ) {
					return false;
				}
			}
		}
	}
	// nothing found return the default value
	return $is_available;
}
add_filter( 'woocommerce_shipping_free_shipping_is_available', 'disable_free_shipping_for_certain_product_categories', 20 ); 

?>