Update custom taxonomy field for ACF plugin to recognise existing terms on existing posts

functions.php

<?php 

add_action('init', 'td_add_special_terms_of_existing_posts_to_custom_field');
function td_add_special_terms_of_existing_posts_to_custom_field()
{
    $postTypes = ['post', 'external_urls', 'videos'];
    $custom_query_args = array(
        'post_type' => $postTypes,
        'posts_per_page' => -1,
        'orderby' => 'post_date_gmt',
        'order' => 'DESC',
    );

    $custom_query = new WP_Query( $custom_query_args );

    if ( $custom_query->have_posts() ) :
        while ( $custom_query->have_posts() ) :

            $custom_query->the_post();
            $terms = get_the_terms(get_the_ID(), 'specials');

            if($terms) {
                $terms = wp_list_pluck( $terms, 'term_id' );
            }

            if($terms) {
                update_field('specials', $terms, get_the_ID());
            }
        endwhile;
    endif;
}