14 files declare their use of FeaturesManagerInterface in Features 8.4

AssignmentConfigureForm.php in modules/features_ui/src/Form/AssignmentConfigureForm.php

AssignmentExcludeForm.php in modules/features_ui/src/Form/AssignmentExcludeForm.php

AssignmentFormBase.php in modules/features_ui/src/Form/AssignmentFormBase.php

ConfigurationItemTest.php in tests/src/Unit/ConfigurationItemTest.php

features.drush8.inc in drush/features.drush8.inc

Features module drush integration.

FeaturesAssignmentAlter.php in src/Plugin/FeaturesAssignment/FeaturesAssignmentAlter.php

FeaturesAssignmentExisting.php in src/Plugin/FeaturesAssignment/FeaturesAssignmentExisting.php

FeaturesAssignTest.php in tests/src/Kernel/FeaturesAssignTest.php

FeaturesCommands.php in src/Commands/FeaturesCommands.php

FeaturesDiffForm.php in modules/features_ui/src/Form/FeaturesDiffForm.php

FeaturesEditForm.php in modules/features_ui/src/Form/FeaturesEditForm.php

FeaturesExportForm.php in modules/features_ui/src/Form/FeaturesExportForm.php

FeaturesManagerTest.php in tests/src/Unit/FeaturesManagerTest.php

FeaturesUIController.php in modules/features_ui/src/Controller/FeaturesUIController.php