The maintenance license provides technical support and software updates. You'll still be able to use Synergy after the maintenance license expires, and continue to use whatever version of Synergy you last downloaded, forever. The download page will always show that version.
After your maintenance license expires, you will continue to have unlimited lifetime download access to the version of Synergy you purchased, so there is no obligation to extend your maintenance license.
This does not apply to Synergy Business, which is an annual subscription.
If you have any questions about technical support or software updates, please let us know.