Apparently there were a few changes to the Syncthing command line, requiring changes for Syncthing to start. The current plugin does not work as-is. While I don’t claim this is the best change, below was what I did to syncthing-service at /storage/.kodi/addons/service.system.syncthing/bin/ to get mine working:
#!/bin/sh
#SPDX-License-Identifier: GPL-2.0
#Copyright (C) 2016-present Team LibreELEC
. /etc/profile
oe_setup_addon service.system.syncthingexport TZ=“America/New_York”
sleep 60
STNODEFAULTFOLDER=“1” syncthing --home=$ADDON_HOME
–gui-address=“0.0.0.0:8384”
–logflags=0
–logfile=$ADDON_HOME/syncthing.log
–no-browser
–no-restart
&