how to install and use synergy 2.0 on arch linux, when it is no longer open source?
Your issue is different than mine. I run BSPWM on :0.
$ echo $DISPLAY
From what I've read, Synergy2 currently only likes DISPLAY :0 by default.
You can change it by setting DISPLAY=:1 in the /usr/lib/systemd/system/synergy.service as mentioned in this thread.
Also these specific lines:
These lines are due to the $HOME/.Xauthority as I mentioned.
You can read more at this link.