This is the first stable release of ILIAS version 6. You find more information about the release at https://docu.ilias.de/goto_docu_pg_122177_35.html.
Please use only the files ILIAS-6.0.tar.gz or ILIAS-6.0.zip for download and installing ILIAS and ignore the files offered by GitHub (called 'Source Code') to prevent problems with composer.