Use this code snippet to change the product button link on out of stock products WooCommerce on child sites.
Snippet Type
Execute on Child Sites
Snippet
add_filter( 'woocommerce_product_add_to_cart_url', 'out_of_stock_read_more_url', 50, 2 );
function out_of_stock_read_more_url( $link, $product ) {
// Only for "Out of stock" products
if( $product->get_stock_status() == 'outofstock' ){
// Here below we change the link
$link = home_url("/");
}
return $link;
}