Support Serial Field

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.

I think the most important would be link fields:

1. having the option to show the link under the code would be great, so thatusers with the scanning device can scan it, but others (say mobile phone users, when showing a QR code) may directly click the link to reach the same destination;
2. having the option to use custom protocols (for deep links) would be great too, ie. not only https:/ but myapp:/ or any other (whitelisted from settings maybe) open source protocol

This issue outlines how to extend the Barcodes module to support additional field types. The link field is already supported, and so is displaying the encoded text below the barcode. The template can be used to explicitly position or style the text, including styling as a clickable link.

Custom protocols should really be implemented as part of the field definition - the FieldFormatter (Barcodes) is only involved with displaying the field contents, and does not participate in validating or constraining the contents of the field. That is done by the FieldType and the FieldWidget.

You can extend the core Link field to support additional protocols by implementing hook_field_info_alter() and changing the field constraints. See core/modules/workspaces/src/Hook/EntityTypeInfo.php for an example.