I still have several machines being backed up with rdiff-backup. It still works great. And it restores, too! Did I mention that you can get your files back in case of a disaster?
It backs up and it restores. It's powerful enough that I'd describe it as "Time Machine on the command line" if I weren't afraid of Apple suing me. If there are no features to add and no bugs to fix, then what possible update could one release? Sure, I suppose you could add a GUI, but that would be a separate project...
In short, no, there's no real need for concern here.
If you really find this a cause for concern, try contacting the developer and asking why there haven't been any updates recently.
Keep in mind that it's also free software, so if the developer isn't responsive, or died four years ago, or whatever, anyone can pick up the project and run with it. It wouldn't be the first time such a thing has happened.