Add text to Out of Stock variations in WooCommerce - https://aceplugins.com/showing-cart-and-checkout-savings-woocommerce/

add-out-of-stock-text-variations-woocommerce.php

<?php // For implementation instructions see: https://aceplugins.com/how-to-add-a-code-snippet/	

/**
 * Add text to out of stock variations.
 *
 * Related post: https://aceplugins.com/showing-cart-and-checkout-savings-woocommerce/
 */
function ace__add_text_out_of_stock_variations() {
	wp_add_inline_script( 'wc-add-to-cart-variation', "
		var outOfStockText = ' (out of stock)'
		jQuery('.variations_form').on('woocommerce_update_variation_values', function() {
			jQuery(this).find('option:disabled').text(function(i, v) {
				return v.replace(outOfStockText, '') + ' ' + outOfStockText
			})
		})
	" );
}
add_action( 'wp_enqueue_scripts', 'ace__add_text_out_of_stock_variations' );