Bootstrap 4 extension version 2.0.6 was released. In this version there are two changes.

First, documentation on how to migrate from Bootstrap 3 was added.

Second, simplified syntax for progress bars that was present in Bootstrap 3 was brought back:

echo Progress::widget([
     'percent' => 65,
     'barOptions' => ['class' => 'progress-bar-danger']
]);

Thanks to @simialbi for both changes.

See CHANGELOG for details.