Funny, just the other day I was wondering how they deal with the fact that this same subdomain is used for image uploads. That seemed like a security risk to allow user-generated content on the same domain where you could log into Reddit. So maybe that's why they removed it?
Also, obligatory pronouncement that if they ever remove old.reddit.com I will never be back (well, ok, I might use it through Apollo, but I certainly won't be browsing through any of their web interfaces).