The story behind that route might be interesting... See, originally Stack Overflow didn't have passwords - all logins were done via OpenID, so any credential management you'd need to do was done through your provider (Google, LiveJournal, myOpenID, etc) This made account recovery assistance pretty simple: given a verified email address, the system would just send that address an email that reminded the owner of any and all OpenID providers that they'd associated with their account. From there, it was up to the account owner to work with a provider to do things like reset passwords.
Skip forward a few years, and Stack Overflow had its own OpenID provider - now you could sign up with an email and password just like a normal site, except really you were creating an account on https://openid.stackexchange.com/ - so the recovery process remained pretty much the same, just with a new provider that happened to be run by the same company.
So far so good... Except, this was awkward to explain to folks. Really, that was what ended up killing OpenID: folks wanted a "Google" or "Facebook" button, not a whitepaper on fancy new authentication systems.
At this point Stack Overflow decided to try to streamline the login process, making signing up and logging in with their own provider seamless: no need to know anything about OpenID. Now recovery emails started including password reset links, and also reduced or removed information on other OpenID providers that were associated with the account in an effort to reduce confusion. The decision tree for generating those emails got complex.
And the decision tree for supporting users got complex as well. Support staff got frustrated; they'd been used to knowing what would and wouldn't be in a recovery email, and had a pile of templates ready to help folks navigate login issues based on that. But now they were getting replies back from folks who were confused and upset because their recovery email didn't contain information that the support person had asserted it would!
This was the genesis of the vulnerable route: a way for support staff to ensure that they were providing accurate information to users about how they could recover their accounts. By the time of this attack, it was already obsolete; the login system had been redesigned twice since the confusing and complex system that first required it. It was vestigial and forgotten... The ideal breeding ground for vulnerabilities.
(source: I worked at Stack Overflow through the time period described in this post, and was involved in support during the period when the relevant route was useful)