Say it isn't so. After 18 years Yahoo has decided to "delete all content ever posted to Yahoo Groups" and shut the service down. I hope that someone will maintain an archive of the CISSPforum for historical reasons. After December 14th is will disappear into the ether forever. Here's the official announcement.
The current incarnation of CISSPforum does not seem to be going anywhere. The one to which you link is the current one. It is hosted on groups.io and is run by a few members, without any official support or blessing by (ISC)², although they have expressed their tolerance for it.
The prior incarnation of CISSPforum was run by (ISC)² and hosted on Yahoo Groups, but it was deleted 18 months ago when (ISC)² closed it down.
Regarding a historical archive, you might consider joining the new CISSPforum and asking there. Some people are really good packrats and might just have a stashed .mbox.
Incidentally, there is a relationship between groups.io and Yahoo Groups in that they have a common author, Mark Fletcher.
Well, if you read the details, you learn that Yahoo! Groups is not shutting down completely. The company is just changing it from a robust forum with deep archives to a simple e-mail group service. They are deleting all archived messages and files, and will no longer accept upload of files, images, etc. They also will no longer archive even recent e-mails. They will continue to support administrated e-mail communication group lists. Thus, they are back to a simple listserve service, with no historic archive. I guess group administrators and members will be responsible for setting up their own archiving capability, if they actually need such.