Are you sure about that? F-Droid only allows OSS software and old Sync was proprietary, and actually strictly locked down and DRMd (it was one of the few pieces of software I’d always have to fully reinstall when switching custom ROMs because of the DRM). The only thing I could imagine would be a custom repository added to the F-Droid client, that has pirated software.
I̶ w̶a̶s̶ p̶r̶e̶v̶i̶o̶u̶s̶l̶y̶ u̶s̶i̶n̶g̶ S̶y̶n̶c̶ v̶i̶a̶ F̶-̶D̶r̶o̶i̶d̶ a̶s̶ m̶y̶ m̶o̶b̶i̶l̶e̶ r̶e̶d̶d̶i̶t̶ c̶l̶i̶e̶n̶t̶, s̶o̶ I̶ w̶a̶s̶ a̶s̶s̶u̶m̶i̶n̶g̶ i̶t̶ w̶o̶u̶l̶d̶ b̶e̶ a̶n̶ o̶p̶t̶i̶o̶n̶ f̶o̶r̶ t̶h̶e̶ L̶e̶m̶m̶y̶ v̶e̶r̶s̶i̶o̶n̶, b̶u̶t̶ I̶ h̶a̶v̶e̶n̶’t̶ s̶e̶e̶n̶/h̶e̶a̶r̶d̶ o̶n̶e̶ w̶a̶y̶ o̶r̶ t̶h̶e̶ o̶t̶h̶e̶r̶. edit - I was wrong. I must have got Sync from the Playstore.
Are you sure about that? F-Droid only allows OSS software and old Sync was proprietary, and actually strictly locked down and DRMd (it was one of the few pieces of software I’d always have to fully reinstall when switching custom ROMs because of the DRM). The only thing I could imagine would be a custom repository added to the F-Droid client, that has pirated software.