Do non-smartphones exist that are able to sync with any OS after Mountain Lion?
Sync Mate 4 was best to support Nokia S40 sync over Bluetooth.

Yet it is not working under 10.9 or 10.10. And their new version 5 dropped support for S40 altogether.

So going further probably the only way is to setup Virtual Machine with older OS of your choosing and run it from time to time just to sync.