Resilio Sync has long been a plug-in for FreeNAS. It isn't any longer. Here are screenshots of where one would expect to see the plug in.
In the FreeNAS Reddit, this was noted: "The packages were broken / absent in upstream FreeBSD repo. That will need to be fixed at some point so we can re-enable it."
But it's been weeks and this hasn't been fixed, so this appears to be purposeful; if it is, then it _looks_ like this is related to the "servers" discussion that happened here in the forum.