While the project itself is open source, there is a non-free release available to monetary donors that includes a few additional features. The package for Devuan is the GPL release.
The FreeFileSync project does not publish a source tree. While the source is available in a standard tarball, no diffs are presented between versions. I participate in a group that does track the changes in code between FreeFileSync releases.
What’s really interesting to note is that the build available on the OBS uses gtk3, which has an instability in some of the icon functions. Unfortunately Debian, the upstream of Devuan GNU+Linux, has removed some of the more-stable gtk2 libs because gtk2 is really old.
If you are looking to compile FreeFileSync on Devuan with gtk2 bindings, you need these exact packages which are probably available at the Debian archive site.