If so, who has access to it? I don't see a link.

I also don't see lists that are as long as I remember in some forums. That prompted the question.

I'm not aware of any FMT archive at the moment, and neither am I sure exactly what you mean by such an archive (i.e., what it contains). Perhaps you can be more specific.

"Retired" threads.

Maybe it's just a poor memory, but it seems like some have disappeared.

By default, each forum's index page is set to display topics active in the last month. Since the posting activity at FTM has been much quieter in the past month than in the first couple of months, there may literally be fewer threads displayed in that default list than in the same forum, say, six weeks ago.

You can display a different time frame's worth of posts by making another choice in the Show Topics pop-up menu at the bottom left of a forum's index page.

Hmm. Is there a way to make a permanent change?

AFAIK, not in this version of the software. I believe that feature has been requested over at UBBCentral, but I have no idea where it might rank on the developers' list of priorities.