There is some issues in JIRA saying XStream should use platform line ending. The resolution is always set to won't fix and commenter says to use the newLine parameter in the constructor of PrettyPrintWriter.
However, it seems that it is not possible to pass this parameter using a non-deprecated constructors. Any non-deprecated constructor does not accept the newLine parameter.
If passing newLine is really deprecated and the only way is to override PrettyPrintWriter#getNewLine method, then I think there should be a comment in the constructor (or the class) telling this information.