Add custom (ACF) fields to the WordPress Rest Api. https://since1979.dev/add-custom-acf-fields-to-the-wp-rest-api/

get-related-posts-api-field.php

<?php
/**
 * get_related_posts_api_field.
 *
 * Callback function for register_related_posts_api_field
 * Fetch and return the value of a related_posts Acf field.
 *
 * @see https://since1979.dev/add-custom-acf-fields-to-the-wp-rest-api/
 * @uses get_field() https://www.advancedcustomfields.com/resources/get_field/
 */
function get_related_posts_api_field($post)
{
    return get_field('related_posts', $post['id']);
}
?>