WooCommerce display product stock availability status without managing stock level

functions.php

<?php 

add_filter('woocommerce_get_availability', 'my_availability_message', 10, 2);
function my_availability_message($args, $product){

	$args = array(
		'availability' => __('Always Available', 'textdomain'),
		'class' => 'always-available'
	);

	return $args;

}