The olefile module is no longer a required dependency when installing Pillow. Support for plugins requiring olefile will not be loaded if it is not installed. This allows library consumers to avoid installing this dependency if they choose. Some library consumers have little interest in the format support and would like to keep dependencies to a minimum.
Further, the vendored version was removed in Pillow 4.0.0 and replaced with a deprecation warning that PIL.OleFileIO would be removed in a future version. This warning has been upgraded to an import error pending future removal.