WordPress - Custom Post Type and ACF -> REST API: add "acf" property to REST API for posts, pages, and custom post types; include the following code blocks in wp-includes/functions.php

functions.php

/**
 * Register Activities as a custom post type
 */
function cpt_register_activities() {
  $labels = array(
    "name" => __( "Activities", "twentytwenty" ),
    "singular_name" => __( "Activity", "twentytwenty" ),
    "menu_name" => __( "Activities", "twentytwenty" ),
    "all_items" => __( "All Activities", "twentytwenty" ),
    "add_new" => __( "Add New Activity", "twentytwenty" ),
    "add_new_item" => __( "Add New Activity", "twentytwenty" ),
    "edit_item" => __( "Edit Activity", "twentytwenty" ),
    "new_item" => __( "New Activity", "twentytwenty" ),
    "view_item" => __( "View Activity", "twentytwenty" ),
    "view_items" => __( "View Activities", "twentytwenty" ),
    "search_items" => __( "Search Activities", "twentytwenty" ),
    "not_found" => __( "No Activities found", "twentytwenty" ),
    "not_found_in_trash" => __( "No Activities found in trash", "twentytwenty" ),
    "featured_image" => __( "Featured image for this activity", "twentytwenty" ),
    "set_featured_image" => __( "Set featured image for this activity", "twentytwenty" ),
    "remove_featured_image" => __( "Remove featured image for this activity", "twentytwenty" ),
    "use_featured_image" => __( "Use featured image for this activity", "twentytwenty" ),
    "archives" => __( "Activity archives", "twentytwenty" ),
    "insert_into_item" => __( "Insert into activity", "twentytwenty" ),
    "uploaded_to_this_item" => __( "Uploaded to this activity", "twentytwenty" ),
    "filter_items_list" => __( "Filter activities list", "twentytwenty" ),
    "items_list_navigation" => __( "Activities list navigation", "twentytwenty" ),
    "items_list" => __( "Activities list", "twentytwenty" ),
    "attributes" => __( "Activities Attributes", "twentytwenty" ),
    "name_admin_bar" => __( "Activity", "twentytwenty" ),
    "item_published" => __( "Activity published", "twentytwenty" ),
    "item_published_privately" => __( "Activity published privately", "twentytwenty" ),
    "item_reverted_to_draft" => __( "Activity reverted to draft", "twentytwenty" ),
    "item_scheduled" => __( "Activity scheduled", "twentytwenty" ),
    "item_updated" => __( "Activity updated", "twentytwenty" )
  );
  $args = array(
    "labels" => $labels,
    "description" => "Add an ambassador activity / contribution.",
    "public" => true,
    "publicly_queryable" => true,
    "show_ui" => true,
    "show_in_rest" => true,
    "rest_base" => "activities",
    "rest_controller_class" => "WP_REST_Posts_Controller",
    "has_archive" => false,
    "delete_with_user" => false,
    "exclude_from_search" => false,
    "hierarchical" => false,
    "rewrite"   => array( "slug" => "activity" ),
    "query_var" => "activities",
    "menu_position" => 5,
    "menu_icon" => "dashicons-megaphone",
    "supports" => array( "title", "custom-fields" )
  );
  register_post_type( "activities", $args );
}
add_action( 'init', 'cpt_register_activities', 0 );




/**
 * Add "acf" property to REST API get and put
 * To support post, need to use https://github.com/airesvsg/acf-to-rest-api
 * From https://stackoverflow.com/questions/56473929/how-to-expose-all-the-acf-fields-to-wordpress-rest-api-in-both-pages-and-custom
 */
function create_ACF_meta_in_REST() {
  $postypes_to_exclude = ['acf-field-group','acf-field'];
  $extra_postypes_to_include = ["page"];
  $post_types = array_diff(get_post_types(["_builtin" => false], 'names'),$postypes_to_exclude);

  array_push($post_types, $extra_postypes_to_include);

  foreach ($post_types as $post_type) {
      register_rest_field( $post_type, 'acf', [
          'get_callback'    => 'expose_ACF_fields',
          'update_callback' => 'expose_ACF_fields',
          'schema'          => null,
      ]
    );
  }
}
function expose_ACF_fields( $object ) {
  $ID = $object['id'];
  return get_fields($ID);
}
add_action( 'rest_api_init', 'create_ACF_meta_in_REST' );