15 files declare their use of PluginBase in Backup and Migrate 8.4

CompressionFilter.php in lib/backup_migrate_core/src/Filter/CompressionFilter.php

DatabaseSource.php in lib/backup_migrate_core/src/Source/DatabaseSource.php

DBExcludeFilter.php in lib/backup_migrate_core/src/Filter/DBExcludeFilter.php

DestinationBase.php in lib/backup_migrate_core/src/Destination/DestinationBase.php

DrupalBrowserUploadDestination.php in src/Destination/DrupalBrowserUploadDestination.php

DrupalEncrypt.php in src/Filter/DrupalEncrypt.php

DrupalSiteArchiveSource.php in src/Source/DrupalSiteArchiveSource.php

DrupalUtils.php in src/Filter/DrupalUtils.php

FileDirectorySource.php in lib/backup_migrate_core/src/Source/FileDirectorySource.php

FileExcludeFilter.php in lib/backup_migrate_core/src/Filter/FileExcludeFilter.php

FileNamer.php in lib/backup_migrate_core/src/Filter/FileNamer.php

MetadataWriter.php in lib/backup_migrate_core/src/Filter/MetadataWriter.php

Notify.php in lib/backup_migrate_core/src/Filter/Notify.php

SourceBase.php in lib/backup_migrate_core/src/Source/SourceBase.php

StreamDestination.php in lib/backup_migrate_core/src/Destination/StreamDestination.php