Some sites have gone some significant changes of scope policy over the years. At one time the homework tag was policy on Stack overflow. It isn't anymore. And then there's the entire early history of Not Programming Related Software Engineering.
I'm sure many metas have these old polices sitting around. These old policy pages can often confuse users coming to meta and looking for is something on topic. On Software Engineering.SE, I had a user quoting a page from 2011 about the on-topicness of career questions. It's 2015 now, and career questions are categorically off topic because the experiments of them being on topic in 2011 (and on) really failed to produce good questions or answers (they kept getting asked again and again with minor variations ('the other post was about a 21 year old, I'm 25')... anyways, that's stuff of M.Software Engineering.SE.
Without being revisionist of the history - because that causes confusing comments and not quite answers to questions, create a special lock for meta sites.
This question exists because it has historical significance about the development of the scope of {site}. However, this scope has changed over time and the reader is urged to seek more recent posts on this topic.
Another variation / suggestion:
This question exists because it has historical significance, but it is not considered representative of the current policies of this site. This question and its answers are frozen and cannot be changed. More info: help center
This way mods can properly identify old policy posts and lock them.
Why mods? Why not just have users dup them to the new one? Mostly because it puts too much power in the hands of the community for being revisionist. We can go all over and silently duplicate things to other things (because how many 10k users actually watch the meta 10k tools?) and also delete historical policies that we don't particularly like.
Locking them freezes the votes, prevents duplication and commentary (there've also been situations where someone comments on an ancient policy and tires stirring up discussion on something we've put behind us for years on that post).
It is important to record where we've been and keep that around. The discussions and meta posts from '10, '11, '12, '13 and '14 have brought us to where we are now. For someone who wants to understand why certain questions are off topic on Software Engineering.SE or how best to ask a certain type of question that frequently comes up (and has been given various meta posts of guidance), it is important to see the gradual changes over time as various policies were attempted and modified later, or wholesale updates and clarifications to policies. Otherwise, meta is even more confusing than it is normally to new users.