馃帄 Happy Syncthing 2.0.0 day to those who celebrate it.
Upgrading on a Mac was as easy as a homebrew upgrade, stopping the old service, running syncthing by hand to migrate the database, and then restarting the service.
https://github.com/syncthing/syncthing/releases/tag/v2.0.0
Update: I had to brew remove syncthing and then brew install syncthing before restarting the service, before Homebrew would replace the outdated service command flags/options.