Redirect Old pages to Home ( Same Website, not different domain)
https://webmasters.stackexchange.com/questions/61466/htaccess-redirecting-404-urls-to-index-page
Put this in the Wordpress 404.php page, at the very top
<?php
header("HTTP/1.1 301 Moved Permanently");
header("Location: ".get_bloginfo('url'));
exit();
?>