7 files declare their use of BackupFileWritableInterface in Backup and Migrate 8.4

ArchiveWriterInterface.php in lib/backup_migrate_core/src/Service/ArchiveWriterInterface.php

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

DrupalEncrypt.php in src/Filter/DrupalEncrypt.php

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

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

TarArchiveWriter.php in lib/backup_migrate_core/src/Service/TarArchiveWriter.php

TempFileManagerInterface.php in lib/backup_migrate_core/src/File/TempFileManagerInterface.php