This thread isn't really about what people did back in the day. When Ansel mounted prints they didn't know a whole lot of squat about archivability. It was just getting going at that point. People mounted their prints on straight cardboard. I believe Weston did this and remarkably many have held up. I just recently saw a Weston print of Tina on the Azotea at Art Basel Miami that had yellowing around the edges, so that one, not so much. Probably was mounted or framed with something non archival. The point is if you mount, you run the risk of later having a problem, or passing the problem on to someone else. Odds are a museum won't want to unmount the print just because it is expensive for them and there is a chance that the print might get damaged. In other words, what people do and what museums want are two different things. As others have suggested though, as the size of the print increases there becomes no real alternative to mounting the print. Photo corners on a large print is possible, but the expansion and contraction of the print doesn't do the display of it any favors.
The times I have tried to unmount a dry mounted print ended up being a pain. The print came off the matt fine, but getting the mounting tissue off the print was practically impossible. That is why I don't mount prints anymore. I don't print big though, so it is an option for me. Not that anyone cares about my prints anyway.....