Filters and example code for Yoast SEO robots or WP robots.txt

custom_robots.php

<?php
/********* DO NOT COPY THE PARTS ABOVE THIS LINE *********/

/*
 * Replace Disallow with Allow Generated Robots.txt
 * Credit: Unknown
 * Last Tested: Unknown
 */

add_filter('robots_txt','custom_robots');

function custom_robots($output) {
     $public = get_option( 'blog_public' );
     if ( '0' != $public )
     return str_replace('Disallow','Allow',$output);
}

yoast_seo_robots_remove.php

<?php
/********* DO NOT COPY THE PARTS ABOVE THIS LINE *********/

/*
 * Remove meta robots from Yoast SEO
 * Credit: Yoast development team
 * Last Tested: Mar 01 2017 using Yoast SEO 4.4 on WordPress 4.7.2
 */

add_filter( 'wpseo_robots', '__return_false' );

yoast_seo_robots_remove_search.php

<?php
/********* DO NOT COPY THE PARTS ABOVE THIS LINE *********/

/*
 * Change meta robots using Yoast SEO
 * Credit: Yoast development team
 * Last Tested: Jun 19 2017 using Yoast SEO 4.9 on WordPress 4.8
 */

add_filter( 'wpseo_robots', 'yoast_seo_robots_remove_search' );

function yoast_seo_robots_remove_search( $robots ) {
  if ( is_search() ) {
    return false;
  } else {
    return $robots;
  }
}

yoast_seo_robots_remove_single.php

<?php
/********* DO NOT COPY THE PARTS ABOVE THIS LINE *********/

/*
 * Change meta robots using Yoast SEO
 * Credit: Yoast development team
 * Last Tested: Dec 12 2017 using Yoast SEO 9.2.1 on WordPress 5.0
 *********
 * DIFFERENT POST TYPES
 * Post: Change 123456 to the post ID
 * Page: Change is_single to is_page and 123456 to the page ID
 * Custom Post Type: Change is_single to is_singular and 123456 to the 'post_type_slug'
    Example: is_singular( 'cpt_slug' )
 *********
 * MULTIPLE ITEMS
 * Multiple of the same type can use an array.
    Example: is_single( array( 123456, 234567, 345678 ) )
 * Multiple of different types can repeat the if statement
 *********
 * The return false removes the robots tag on the page
 * Or you can return index/noindex follow/nofollow like
 * return 'noindex,follow';
 * Or
 * return 'noindex,nofollow';
 */

add_filter( 'wpseo_robots', 'yoast_seo_robots_remove_single' );

function yoast_seo_robots_remove_single( $robots ) {
  if ( is_single ( 123456 ) ) {
    return false;
  } else {
    return $robots;
  }
}