10 files declare their use of PluginBase in Express 8

ElementInfo.php in themes/contrib/bootstrap/src/Plugin/Alter/ElementInfo.php

Contains \Drupal\bootstrap\Plugin\Alter\ElementInfo.

FormBase.php in themes/contrib/bootstrap/src/Plugin/Form/FormBase.php

Contains \Drupal\bootstrap\Plugin\Form\FormBase.

LibraryInfo.php in themes/contrib/bootstrap/src/Plugin/Alter/LibraryInfo.php

Contains \Drupal\bootstrap\Plugin\Alter\LibraryInfo.

PageAttachments.php in themes/contrib/bootstrap/src/Plugin/Alter/PageAttachments.php

Contains \Drupal\bootstrap\Plugin\Alter\PageAttachments.

PreprocessBase.php in themes/contrib/bootstrap/src/Plugin/Preprocess/PreprocessBase.php

Contains \Drupal\bootstrap\Plugin\Preprocess\PreprocessBase.

ProcessBase.php in themes/contrib/bootstrap/src/Plugin/Process/ProcessBase.php

Contains \Drupal\bootstrap\Plugin\Process\ProcessBase.

ProviderBase.php in themes/contrib/bootstrap/src/Plugin/Provider/ProviderBase.php

Contains \Drupal\bootstrap\Plugin\Provider\ProviderBase.

SettingBase.php in themes/contrib/bootstrap/src/Plugin/Setting/SettingBase.php

Contains \Drupal\bootstrap\Plugin\Setting\SettingBase.

ThemeSuggestions.php in themes/contrib/bootstrap/src/Plugin/Alter/ThemeSuggestions.php

Contains \Drupal\bootstrap\Plugin\Alter\ThemeSuggestions.

UpdateBase.php in themes/contrib/bootstrap/src/Plugin/Update/UpdateBase.php

Contains \Drupal\bootstrap\Plugin\Update\UpdateBase.