12 files declare their use of HandlerBase in Drupal 9

AreaPluginBase.php in core/modules/views/src/Plugin/views/area/AreaPluginBase.php

ArgumentPluginBase.php in core/modules/views/src/Plugin/views/argument/ArgumentPluginBase.php

FieldPluginBase.php in core/modules/views/src/Plugin/views/field/FieldPluginBase.php

FilterPluginBase.php in core/modules/views/src/Plugin/views/filter/FilterPluginBase.php

HandlerAllTest.php in core/modules/views/tests/src/Functional/Handler/HandlerAllTest.php

HandlerBaseTest.php in core/modules/views/tests/src/Unit/Plugin/HandlerBaseTest.php

Contains \Drupal\Tests\views\Unit\Plugin\HandlerBaseTest.

HandlerTest.php in core/modules/views/tests/src/Functional/Handler/HandlerTest.php

ManyToOneHelper.php in core/modules/views/src/ManyToOneHelper.php

RelationshipPluginBase.php in core/modules/views/src/Plugin/views/relationship/RelationshipPluginBase.php

SortPluginBase.php in core/modules/views/src/Plugin/views/sort/SortPluginBase.php

Sql.php in core/modules/views/src/Plugin/views/query/Sql.php

ViewsHandlerManager.php in core/modules/views/src/Plugin/ViewsHandlerManager.php