Grey Out Out-of-Stock Variations

Makes sold-out variations unselectable in the product options dropdowns, so customers cannot pick a size or colour you no longer have.

Code

add_filter( 'woocommerce_variation_is_active', function ( $active, $variation ) {
    return $active && $variation->is_in_stock();
}, 10, 2 );

Works best for products with up to 30 variations, where WooCommerce loads all variation data on the page.

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