When trying to install Synergy's latest version (1.11+) on macOS versions older than 10.15 (Catalina):
OS X 10.10: Yosemite
OS X 10.11: El Capitan
macOS 10.12: Sierra
macOS 10.13: High Sierra
macOS 10.14: Mojave
You will receive an error "The following disk images couldn't be opened: Reason: no mountable file systems"
This is because the installer for macOS 10.15 changed because of the Notarization requirement from Apple.
For versions from macOS 10.10 through 10.14, you can still install the legacy version (1.8.8 or 1.10.2).