Historical locks are sometimes applied to questions.
A historical lock is a mechanism by which moderators can mark posts as historical artifacts. Questions which are historically locked feature the following post notice:
Locked. This question and its answers are locked because the question is off-topic but has historical significance. It is not currently accepting new answers or interactions.
This is an example of a question with a historical lock: Changes to syntax highlighting
A historical lock preserves content that was very popular when it was originally posted, but is now off-topic or otherwise out of scope for the site it is posted on. Historically locking a post ends the debate over whether a question should be kept on the site or deleted, and is often the final state of a question that has been deleted and undeleted more than once, or subject to a close or delete war.
In addition to the post notice being prominently displayed, posts which are historically locked are "frozen in time": they cannot be voted on, flagged, answered, edited, bountied, or commented on (though they can be bookmarked, and ♦ moderators can edit and comment; moderator edits to historically locked posts do not bump the posts). The visual appearance of the entire post is altered by removing the voting arrows from the question and all answers.
Additionally, on main (non-meta) sites, historically locked questions are omitted from normal question lists (those on the home page, /questions, and the various per-tag lists), but can still be found by searching for words in the post or title (via either site-search or Google, etc.) or by using the search operator locked:1.
Questions can be historically locked when:
Questions should not be historically locked if they are being actively maintained or have little or no redeeming value.
A good rule of thumb: If the question does not minimally meet site co-founder Jeff Atwood's third rule in the "We Hate Fun Here" blog post, it's probably not a good candidate for historical locking. The third rule is:
Does this question teach me anything that could make me better at my job? Can I learn something from it?
Flag the question for moderator attention and explain why you think the post should be historically locked. A moderator will evaluate the question using the criteria outlined above and will either lock the question or decline your flag with an explanation.
Alternatively, if you want to try and gain more community support for the question, or contest the moderator decision, you can post a question on the per-site meta.
The system prevents you from being able to flag historically-locked posts; this is because these posts have tended to get lots of frivolous flags in the past. Therefore, if you notice a serious problem with a historically-locked post (e.g., a broken link that has since started serving spam or malware, content that is blatantly offensive, or substantiated requests to have a historical lock removed), post a question on the per-site meta explaining the exact problem. (If the locked question is here on Meta Stack Exchange itself, post your request here on this site with the tag meta.)
You should reserve this for serious issues like the ones above, rather than minor issues like grammar fixes or things that go against the rationale for historical locks.
If the issue is non-controversial, easily explained, and easily rectified (e.g. the broken link or blatantly offensive content mentioned above), you can alternatively raise an "in need of moderator intervention" flag on another post (usually one of your own posts). In your flag, be sure to link to the post you're actually flagging about and explain both the issue and how you feel it should be corrected.
If there was already a bounty on a question before it got historically locked, it will not be cancelled, but the bounty owner cannot award the bounty to anyone. It can still be automatically awarded, though. If there are no answers eligible for automatic awarding, consider letting a moderator know to clear the bounty to remove the question from the list of bountied questions.
While no new votes can be cast and users cannot retract their past votes, scores on historically-locked posts can still change as a result of vote invalidations, such as the serial vote detection script reversing votes, manual voting invalidations by staff, or a user who voted on the post being removed (and their votes weren't preserved).