The next major update to Viscosity, version 1.6, will end support for Mac OS X 10.7 (Lion) and Windows XP. The current release, version 1.5.11, will continue to function on these platforms however it will no longer receive updates and security fixes. Version 1.6 is expected to be released later this month.
The decision was made as these platforms are no longer actively supported by Apple and Microsoft, they are missing critical security updates, and only a very small fraction of Viscosity's user base (<0.1%) are still using these platforms.
Version 1.5.11 of Viscosity will still be available for users from the Legacy Downloads page, however if you're still on one of these unsupported platforms we encourage you to upgrade to a newer operating system version to ensure that you continue to receive the latest updates.