You'll still be able to use Synergy, and you'll have access to what you paid for, forever. 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.
Optionally, if you want to, you can purchase an extended maintenance license for $9.00 USD per year. Both personal editions of Synergy include a maintenance license, which gives you access to software updates and technical support. Depending on the edition you purchase, Synergy includes either a 1 year or 5 year maintenance license as standard. This does not apply to the Business edition.