Instead of being called every time to check what files are on the server , this function can be called manually or be set in auto watch.
This is a simple retention as it does not use the file number but the file date.
UE should compare the first file date on the server and the first file date on your newsgroup. If any file on your newsgroup is older than that on the server, then it is removed.
If you have more than one server for a newsgroup then you can set an option of "1 retention" or "ALL retention" for that newsgroup.
"ALL retention" if the file does not exists on all the servers then delete. The cut of point is set to the earliest first file date.
"1 retention" if the file exists on only one keep the file. The cut of point is set to the oldest first file date.
Simple Retention on server side
what you missed you cannot check the earliest file date on the server easily, you need to scan all server headers to get this information, right now i think UE does the best job possible within the protocol limitations to handle retention, I only could optimize it a little comparing to newspro, since there is no much space to go.