Synergy seems pretty dead. It hasn’t been updated forever, so that proves the “paid” open source model sucks 
I recently took ~my~ the synergy-haiku codebase i’ve been managing, and refactored it for Barrier under a new name (Barrier is new, again open-source version with some protocol changes). barrier-haiku is working ~80% for me. Just disable tls.
Barrier (and Synergy) has some issues with wayland desktops on Linux though… ymmv