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.