I was a student at the University of Tennessee at Martin for one semester before I completed my degree later in the 90's and WUTM, the campus radio station had a big box of 45 demos from the 70's that they were giving away. They were mostly ones that were never big hits, but I grabbed some and I think I ended up giving most of them to Goodwill.
On the subject of stations dumping demos with a format change, when WMSO 640, a CCM station in Memphis was sold to Bott Broadcasting in 1986, I was told by a former announcer later that I had contact with that they dumped all the demos because Bott had an attitude of "We don't do music", especially CCM. There may have been some that could have ended up in the hands of people who knew that they had been dumped, but they probably had to do some dumpster diving to get anything. Bott also bought WNAZ in Nashville in 2011, but by that time it was probably more a matter of wiping hard drives unless there were older demos there that were probably dumped unless some former staffers were able to get them.
Also in Memphis KSUD 730, another CCM station was sold to EMF along with their FM on 94.7 around 2000, but stayed local on AM for a few more years. Later they switched the AM to simulcasting the FM until it was sold in 2005 and became a Sports Talk station. Some time after that I had seen on this forum where someone had the demos from KSUD available. I tried to find out more but never heard back. But I'd like to think at least some of those demos were saved.