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This Mumble Moderator Module sends a message to clients, which use a version of their Mumble client below a defined version, so that they know they should update.
In the new version one can change, whether users with old versions should be kicked additionally.
The script is available on Github: https://github.com/Natenom/mumblemoderator-module-collection/tree/master/wrongversion
See Installation of Modules for Mumble Moderator.