What's worse these days is, ever since the Equifax breach, certain institutions have taken to asking me for the last 6 digits of my social (or even worse... all of them...)
SSN isn't even a 'secret' number. Military folk have it on their ID tags and you're supposed to give it up if you're a POW. If you're supposed to give it to your enemies, then how is it private info?
The last 4 or 6 are the ones with the most information. The preceding 3-5 digits are handed out in blocks to the states. So if you roughly know when and where someone is born, you can guess their prefix.