Support additional field types

Works for me. RTBC?

Well, serial isn't one of the built-in Drupal core field types. It is not a scalable solution to keep adding names of contributed field types. I would prefer a more general solution, using FormatterBase::isApplicable() and possibly removing the field_types annotation if it is no longer relevant.

Just uploading a rerolled version for Barcodes 2.0.x.

Like I said, this will not be added to the module. This issue is still active because I'm open to a solution that allows site builders to customize this without hacking or patching code.

The standard way to customize Drupal for your site is to implement a hook. This would work for your needs, for example:

/**
 * Implements hook_field_formatter_info_alter().
 */
function yourmodule_field_formatter_info_alter(array &$info) {
    // Allow the barcode field formatter to be used on a new field type.
    $info['barcode']['field_types'][] = 'serial';
}

And I feel that for reference I need to add that "yourmodule" should be replaced with the name of your module that you are using to customize your site. Read the documentation on how to implement a hook for details.