If you buy the commercial version for the Catalina machine, the easiest hack seems to be:
- Install Synergy on the Catalina Mac and configure it as a server
On the Sierra Mac:
- download and install the 10.13 version on the Sierra Mac. You can install it, but the app won't run
- open a terminal
- Enter the command /Applications/Synergy.app/Contents/MacOS/synergyc -f
name_or_IP_of_catalina_mac
- Should connect - you may need to add a "client" definition with the correct name
...you could, of course, do the last two steps over a SSH connection from the Catalina Mac if you wanted to connect without having to plug in a keyboard. Or use something like Mongoose to create an App that you could add to your startup items. Or write a launchd script, etc.
Also, the old open source version is still on MacPorts (
https://www.macports.org/) - if you're up for the faff of installing MacPorts (which also involves installing XCode) you can install synergy with:
port install synergy
...after which the command to connect it to the server is just:
synergyc
name_or_IP_of_catalina_mac
You can also use the MacPorts version to run a server, but you'll have to write a text-based config file. You can google for instructions.
I tried HomeBrew (which is sometimes friendlier than MacPorts) but it rather unhelpfully just downloads a non-Sierra-compatible binary of the GUI App.
It's a pity that Symless couldn't have provided better support for older versions of MacOS - keeping old Macs going to run legacy software is a major use for synergy.