Still testing this solution (still using 1.9.1), but here's what I did to reconcile Synergy with libcurl4 on Ubuntu 18.04:
Downloaded the appropriate Synergy package.
Unarchived the package, then unarchived the resulting control.tar.gz, all so I can edit the control file:
$ ar x synergy_1.9.1.stable_b327+2a0225c1_amd64.deb
$ tar zxvf control.tar.gz
$ vim control
Modify the control file, removing libcurl3 with libcurl4 (>= 7.58.0)
Package: synergy
Version: 1.9.1.stable~b327+2a0225c1
Architecture: amd64
Maintainer: Symless <engineering@symless.