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

register-related-posts-api-field.php

<?php
/**
 * register_related_posts_api_field.
 *
 * Register a Rest Api field to add the related posts Acf field to the /post endpoint.
 *
 * @see https://since1979.dev/add-custom-acf-fields-to-the-wp-rest-api/
 * @uses register_rest_field() https://developer.wordpress.org/reference/functions/register_rest_field/
 * @uses array() https://www.php.net/manual/en/function.array.php
 */
function register_related_posts_api_field()
{
    register_rest_field('post', 'related-posts',
        array(
            'get_callback' => 'get_related_posts_api_field',
            'schema' => null,
        )
    );
}

/**
 * Hook: after_setup_theme.
 *
 * @uses add_action() https://developer.wordpress.org/reference/functions/add_action/
 * @uses rest_api_init https://developer.wordpress.org/reference/hooks/rest_api_init/
 */
add_action('rest_api_init', 'register_related_posts_api_field');
?>