Most times it's a driver's or operator license number.
However, I hear SSNs quite often around here on state police talkgroups -- used usually when the driver (person, etc.) doesn't have a driver's license or other ID when stopped by police.
In fact, hopefully a rare situation, I recently heard even more dangerous information passed in the clear on a business system talkgroup - credit card specifics (all of it including security code), name, etc.
Definitely not good....
My county police are mostly unencrypted for dispatch/routine calls (as it should be). However, they are smart enough to have encrypted some talkgroups including one to do want/warrant and other checks and operational uses where the information should be protected.
Others around the area - including law enforcement in some local jurisdictions and even state police - haven't gotten that smart (yet). Unfortunately, what will happen is eventually they just encrypt everything using this situation as one of the reasons for doing so.
Years ago back home in southeastern VA it was commonplace to hear SSNs being tossed about over the air consistently but I mean years ago as in the early 80s, just before trunked systems came into being with widespread use. The issue for my state was that long long ago Virginia actually used a person's SSN as their driver's license ID number - they were taken to court over it since technically it is a Federal offense to use an SSN for anything but the purpose it was designed for meaning only the Social Security Administration is supposed to use it for ID purposes of any kind.
At some point in the late 1980s Virginia shifted to using some other type of ID number generation because of that lawsuit but even so these days it seems like EVERYBODY uses an SSN for anything they damned well please when it's not really allowed, it gets done anyway.