Display that a product is in stock even if Track Stock Quantity is not enabled under the product's Inventory tab.

wc-core-in-stock-quanity-not-tracked.php

<?php

add_filter( 'woocommerce_get_availability', 'custom_override_woocommerce_show_in_stock', 10, 2 );

function custom_override_woocommerce_show_in_stock( $availability, $product ) {
	
    if ( ! $product->managing_stock() && $product->is_in_stock() ) {
        $availability['availability'] = __( 'In Stock', 'woocommerce' );
    }
	
    return $availability;
}