v0.9.0 includes the ability to perform upgrades directory from the GUI. The check is performed when the GUI is loaded and if a newer version is available an upgrade button is shown in the top bar.
For those of you building syncthing for environments where this is not desirable (Android, package managers, etc):
./build.sh "" -tags noupgrade or make sure that the Go compiler gets
-tags noupgrade in some other way. This removes both the upgrade check in the GUI and the
syncthing -upgrade option.