The historical notice is good: the other main reason for locking is when an edit war ensues. I'd like to see something like:
This post has been locked while disputes about its content are being resolved. More info: {link}
While it'd be ideal to be able to specify a meta discussion topic or something for the {link}
, a link to the meta site or the FAQ could be a substitute if that's not possible.
The only other reason I see posts get locked, and it's a bit of an edge case, is when the post is controversial and generates a metric ton of comments (example). A message like this could be useful:
This post has been locked due to the high amount of off-topic comments generated. For extended discussions, please use [chat].
We talked about this in chat, but just for the sake of completeness for anyone coming across this in the future, one issue we run into periodically on Programmers are questions that just get an overwhelming number of answers, to the point where people just repeat other answers because nobody bothers to see if one of the 100 other answers is the same as theirs.
As Jeff noted in the comments the system warns people when they're adding an answer to a question that already has a bunch of answer already, and in the past, we've added moderator notices like these to those questions as warnings:
Moderator Message
This question already has nearly N answers. Therefore, please consider whether your answer is going to provide anything new. Please take a moment to review all the answers before posting.
But eventually the question gets locked because nobody bothered to pay attention to the warnings anyway. For these locked questions and for future occurrences, it'd be helpful to have a message like:
This question has an overwhelming number of answers that address every possible aspect of this question. Feel free to improve existing answers. More info: FAQ