I still cannot believe NASA managed to re-establish a connection with Voyager 1.
That scene from The Martian where JPL had a hardware copy of Pathfinder on Earth? That’s not apocryphal. NASA keeps a lot of engineering models around for a variety of purposes including this sort of hardware troubleshooting.
It’s a practice they started after Voyager. They shot that patch off into space based off of old documentation, blueprints, and internal memos.
Modern satellites are protected by various means of encryption, but there’s an enthusiast community that tracks down and communicates with very old unencrypted zombie satellites. There’s even been an NGO which managed to fire rockets on an abandoned NASA/ESA probe (with their approval.)
The Voyagers benefits primarily from the lack of groups with an adequate deep space network to communicate with it. Their communication standards are otherwise completely open and well documented.