Remove meta robots, like "noindex" and "nofollow" for the 404 template, from Yoast SEO

remove-yoast-noindex-for404.php

/********* remove 'noindex' from 404 pages *********/

/* Based on this snippet: 
 * https://gist.github.com/amboutwe/0c71e42aa164238007d7ea88f174a93f#file-yoast_seo_robots_remove_search-php
*/

add_filter( 'wpseo_robots', 'yoast_seo_robots_remove_404' );

function yoast_seo_robots_remove_404( $robots ) {
  if ( is_404() ) {
    return false;
  } else {
    return $robots;
  }
}

This goes in functions.php.