Limit WooCommerce Cart to include only one of the specific products and nothing else

woocommerce-disallow-adding-specific-products-with-others-to-cart.php

<?php


/**
 * Return if product can be added to the cart
 * @param $valid
 * @param $product_id
 * @return bool
 */
function avoid_special_products_mix($valid, $product_id): bool
{
    $current_cart_items_count = WC()->cart->get_cart_contents_count();

    if ($current_cart_items_count > 0) {
        // TODO Put specific product ids
        $unmixable_products_id = [];
        $unmixable_products_cart_ids = [];
        foreach ($unmixable_products_id as $item) {
            $unmixable_products_cart_ids[] = WC()->cart->generate_cart_id($item);
        }
        foreach ($unmixable_products_cart_ids as $item) {
            if (WC()->cart->find_product_in_cart($item)) {
                $valid = false;
                break;
            }
        }
        if (in_array($product_id, $unmixable_products_id)) {
            $valid = false;
        }
        if (!$valid)
            // Show an error in case of adding specific products with others in cart
            wc_add_notice(sprintf(__("Empty <a href=\"%s\">your cart</a> first as selected product must be ordered separately.", "text-domain"), wc_get_cart_url()), 'error');

    }
    return $valid;
}

add_filter('woocommerce_add_to_cart_validation', 'avoid_special_products_mix', 10, 2);