Adds .outofstock class to WooCommerce variation input tag

radio-stock-status.php

add_action( 'woocommerce_after_add_to_cart_form', 'radio_waitlist_label' );
function radio_waitlist_label() {
	echo "
		<script>
		jQuery(document).ready(function($) {
			var variation_data = $('form.variations_form').attr('data-product_variations');
			var variation_data = JSON.parse(variation_data);
			$('.variations td.value div input').each(function() {
				for (var i = 0; i < variation_data.length; i++) {
					var variation = variation_data[i];
					if ($(this).val() == variation.attributes.attribute_pa_size) {
						if ( variation.is_in_stock == false ) {
							$(this).addClass('outofstock');
						}
					}
				}
			});
		});
		</script>";
}

Works with this plugin https://en-ca.wordpress.org/plugins/wc-variations-radio-buttons/

Just style the labels after the .outofstock class. E.g.

.outofstock + label {
    text-decoration: line-through;
    color: #ddd;   
}