MySQL workbench giving an error "Unsupported operating system" when running under Windows 8.1
It means exactly what it says where the language states "it wasn't designed to run on your platform", so unfortunately this appears to be an instance where the unsupported OS version is experiencing a bug and the entire message meaning is applicable.
Workaround
As a workaround, try an older version of MySQL Workbench from MySQL Workbench (Archived Versions) site. Just select an older version from the Previous Version drop down.
Trial and Error
This may have to be a trial and error type of solution so uninstall the version of MySQL Workbench with the bug, reboot, install an older version, and then test. If you run into the same bug with the previous version, keep moving back and following that same process to uninstall, reboot, install, test with each version.
MySQL Workbench (Archived Versions)
Please note that these are old versions. New releases will have recent bug fixes and features!
Product Version: <select the version> Operating System: <select the os>Select the
Downloadbutton once the needed version and OS is selected