Hi Brett,
There's a built in database of DGPS stations, which can be overrided with your own text file if named dgps_station_list.txt
DGPSSTATIONLISTING.TXT.BAK.TXT can be used as a basis for creating that file. Now, I will add... I tested doing that when I first implemented using an external station listing file, but have not tried it since, and I think I had a report from someone that it didn't work, but that was a while go, and I am not sure if they ever resolved it or not. In general, everyone is just using the built in station listing.
The stations in the file with a - are indeed off the air, and there are more since (that was from the big shutdown a few years ago).
The USCG site has a list of stations off the air, I would use it as the official reference. Also check the logs here on the HFU.