With ORNL already having migrated to Red Hat 7 and the migration well underway at RAL, Mantid 3.9 will be the last release of Mantid that will support RHEL 6, future releases will continue with support for RHEL 7.
If for some reason you need to continue using RHEL 6, then versions of Mantid up to and including 3.9 will continue to work, and are available from our Mantid downloads page.