Deskflow is now the upstream project for Synergy.

Deskflow is a free and open source keyboard and mouse sharing app.
Use the keyboard, mouse, or trackpad of one computer to control
nearby computers, and work seamlessly between them. It's like a
software KVM (but without the video). TLS encryption is enabled
by default. Clipboard sharing is supported.

The Deskflow project was established to cultivate
community-driven development where everyone can collaborate.

Synergy sponsors the Deskflow project by contributing code and
providing financial support while maintaining its
customer-oriented code downstream.

More info:
  https://github.com/deskflow/deskflow/wiki/Relationship-with-Synergy

For Slackware 15.0, deskflow is held at version 1.17.0
since later versions require OpenSSL version 3