My understanding is that a question is historically locked because the information in it is too valuable for the question to be deleted.
But if the "valuable information" becomes obsolete, what happens then? We cannot modify the question content, or the answers for a historically locked question, right?
Consider this: posts are historically locked because they are viewed by the community as being immensely useful even though they are glaring examples of what not to do on a Stack Exchange site.
If the answers on a historically locked question are veering towards being (or actually are) not useful anymore, then the justification for the post being historically locked should be revisited. If the value is not there anymore on a historically locked post, then by definition, it shouldn't be historically locked.
Naturally, if it's no longer historically locked, and still a glaring example of a post that is not a good fit for a Stack Exchange site, then it should be deleted.
If this is indeed the case, then you should raise the issue on Meta: it's generally better to get community consensus on these (as they would have been the ones to ask for a historical lock in the first place).
There is really no direct system in place to handle this other than flagging, as casperOne indicated.
However, If such a post:
... the system could conceivably raise an automatic flag to alert moderators that the historical lock might not be appropriate any longer. Still, removing such a hard earned lock is something a moderator is not likely to do without good reason.
What I recommend you do in this case is flag and raise a discussion here questioning the validity of the lock, while clearly stating why some or all content is no longer relevant or perhaps even harmful. The post can then be unlocked, edited and re-locked, or removed with the blessing of a consensus.
I suppose you could flag it, but then you're asking Moderators to make a judgment as to whether the answer is still accurate. The Moderator may not have enough knowledge to effectively make that determination. Even using the "Other" option with a nice long explanation is likely to be insufficient.
I suggest that in such a case it would be an excellent discussion topic for that site's Meta. The community can decide what can be done, whether it's to unlock the question, delete it, edit the answer, or whatever.
