PHPExcel_Writer_Excel5 has evolved from PEAR::Spreadsheet_Excel_Writer.
PEAR::Spreadsheet_Excel_Writer originally saved as Excel 95 (BIFF5), but was later modified to provide a switch setVersion(8) so it could save as Excel 97 / BIFF8.
We have not offered any such switch in PHPExcel_Writer_Excel5. But if you locate this line in PHPExcel\Writer\Excel5.php
$this->_BIFF_version = 0x0600;
and change it to this
$this->_BIFF_version = 0x0500;
Then it should in theory generate BIFF5. Result could be buggy and less feature-rich.
Since you are the first one to ask such question I'm not sure whether we should provide a switch.
Maybe you can explain why you would like BIFF5?