In my opinion, just an opinion as a user, I think you should restrict them from doing that. It will be too confusing. Lots of old users will drop back by, unaware of software changes, and we should try to make everything seamless for them.

If they try /Talk, they surely just mean talk: in the new system. So we can make that transition for them, as transparently as possible?

Other opinions?

I recommend not building in automatic technological restrictions if we believe the combination of a) social pressure and b) automatic ease of doing the "preferred" thing

In other words, having the Talk namespace is great, I think (though it does raise some confusing issues about hierarchies etc.) and we should, if we implement it, automatically convert current /Talk pages to the new system.

That way new people wouldn't have any motivation to create /Talk pages; and old people, who will know what's going on, won't have any motivation to create /Talk pages.

What we don't know, is that in the future, someone might actually have a good reason for wanting a /Talk page. I can't think of one, but I'm not arrogant enough to believe that I can think of every possibility. Someone would need a pretty good justification, because the societal regulations would prevent him from creating the page without good reason.

So I say this is a case in which its unnecessary to build in restrictions in code as long as we make the preferred method the standard and easy to use.