WordPress API V2 – filter and order by ACF field

You could consider using the rest_{$this->post_type}_query filter hook to modify the result of certain REST API queries.

To not mess with existing API calls, you could look at using new query parameters that have not been used. Then, use these parameters in your filter. For example:

GET /wp-json/wp/v2/crowdfunding?per_page=6&foo_parameter=bar

function my_plugin_filter( $args, $request ) {
  if ( $request->get_param( 'foo_parameter' ) ) {
    // Modify $args as needed here.
  }

  return $args;
}
add_filter( 'rest_crowdfunding_query', 'my_plugin_filter', 10, 2 );