The syncthing-service-type has received an incompatible upgrade. This means that if you are already using the Syncthing service, you will have to manually adjust your service declaration.
If your syncthing-configuration declares a value for config-file, then you need to remove ~/.config/syncthing/config.xml, so that it does not exist after reconfiguring. For example:
mv ~/.config/syncthing/config.xml ~/.config/syncthing/config.xml.bak 2> /dev/null
Additionally, you should ensure the *.pem files in ~/.local/state/syncthing/ refer to the desired Syncthing device ID. Early adopters of the config-file field of syncthing-configuration, and people that previously configured Syncthing from ~/.config will have their Syncthing device ID change if they do not move the *.pem files from ~/.config/syncthing to ~/.local/state/syncthing. You can preserve your device ID by running the following command:
cp ~/.config/syncthing/*.pem ~/.local/state/syncthing/
Note that if you specified config-file previously and had your Syncthing device ID change, and would like your original device ID back, then do NOT run the above.
Finally, there are some obscure fields of the various Syncthing records that were renamed (notably, booleans now end in '?'). Refer to the documentation of syncthing-config-file in the Networking Services of the Guix manual to see the new names.