Add CKEditor widget (footnotes) from external addon
It turns out you need to call editor.ui.addButton() to get the buttons to show.

It seems to me that it is not usually needed in vanilla CKEditor development, because it is not included neither in the tutorials nor in the ckeditorfootnotes plugin. However, most of the current Drupal 8 CKEditor modules do include it in the plugin source.

For the simplebox plugin it would read like this, here command is the name of the defined widget.

editor.ui.addButton('SimpleBox', {
    label: Drupal.t('SimpleBox'),
    command: 'simplebox',
    icon: this.path + 'icons/simplebox.png'
});