I’m offering a bounty of 600 US dollars for proper write support to external SD cards (e.g. /storage/extSdCard on my Samsung Galaxy Note 4) in Android 5.0 and later without requiring rooting the device. As per this recent discussion:
doing so appears to require using Android’s Storage Access Framework.
This bounty is payable via PayPal to whoever adds the necessary feature to the Syncthing client for Android, once it’s included in a mainline release (i.e. not a special one-off version), as long as this happens before the end of 2015.
Others who would like to see this feature (I know you’re out there!) are invited to contribute to the bounty.