Automatically add product to cart on visit, if user is not subscribed.

wc-auto-add-product-to-cart.php

<?php

/**
 * Automatically add product to cart on visit
 */
add_action( 'template_redirect', 'add_product_to_cart' );
function add_product_to_cart() {
	if ( ! is_admin() ) {
		$product_id         = 226; //replace with your own product id
		$found              = false;
		$user_subscriptions = wcs_get_subscriptions( array(
			'customer_id' => get_current_user_id(),
			'product_id'  => $product_id,
		) );

		//check if product already in cart
		if ( count( WC()->cart->get_cart() ) > 0 && 0 === count( $user_subscriptions ) ) {
			foreach ( WC()->CART->get_cart() as $cart_item_key => $values ) {
				$_product = $values['data'];
				if ( $_product->get_id() == $product_id ) {
					$found = true;
				}
			}
			// if product not found, add it
			if ( ! $found ) {
				WC()->cart->add_to_cart( $product_id );
			}
		} else {
			// if no products in cart, add it
			WC()->cart->add_to_cart( $product_id );
		}
	}
}