If there is security/critical bug in the version included in Leap 15.1 you should open bug report. If you are interested in having more actual version in Leap 15.2, you should contact syncthing maintainer. The package itself appears actively maintained in network repository:
bor@bor-Latitude-E5450:~$ osc maintainer -e syncthing
Defined in package: network/syncthing
bugowner of syncthing :
-
maintainer of syncthing :
sor.alexei@meowr.ru
bor@bor-Latitude-E5450:~$
For Leap 15.1 it is likely too late, in the meantime you could install more up to date version from network repository.