I don’t think this has anything to do with Canonical or Ubuntu. They just ship the exact same packages as Debian for syncthing.
Looking at Debian’s upload activity, it appears as if uploads of new versions to unstable are made very infrequently: Debian Package Tracker
A bug report requesting an update to 1.23.6 or newer has been left unanswered: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1042364