When dealing with old off-topic questions, should we close or lock them, and why?

Question

Some topics that are now considered off-topic used to be tolerated in the early days of this site. Consequently, there exist old threads with lots of votes/views/answers that are off-topic under current standards. Our practice is to close such threads because keeping them open can send a wrong signal about what is currently on-topic. As @whuber wrote,

[this] has helped us respond to users who complain (sometimes very aggressively) that their off-topic questions should be left open because they are just like such-and-such a question from years ago.

There are two ways to close such old posts. One way is to close them as off-topic as usual. Another way is to "lock" them for "historical significance". Apparently, locking is preferred to closing -- see @whuber's and @Glen_b's comments under Should we close old, outdated and off-topic questions?

I don't know understand why this is the preferred way. I can see a number of disadvantages of locking:

1. It can only be done via flagging, i.e. community cannot take care of locking without mods.
2. Similarly, it can only be undone via flagging, i.e. community cannot reverse a wrong decision without mods.
3. When a thread is locked, it's not possible to edit any answers, post new comments, or even vote on them. I.e. if there is a mistake in one of the answers, it's there forever, and one cannot even comment to point it out. Note that sometimes we are talking about very visible threads with large number of views.
4. The displayed message says that the thread is locked "for historical significance", but often there is no particular significance.

So I see at least four disadvantages (#3 being pretty severe in my opinion), and not a single advantage. What am I missing?

Answer 1

I agree with using a historical lock for the case you've described. It's... kind of exactly the situation the historical lock was invented to handle.

(Terminology note: in this post, when I talk about locks, I'm referring to historical locks only. What I say may not apply to the other kinds of locks we have, like the temporary kind of lock used on posts where comment threads are devolving into unproductive personal attacks.)

At first glance, a lock might seem like a "super-close", but locking and closing are different animals. A locked post is like a museum exhibit: frozen in time and unchangeable, something to just goggle at for a moment before moving on. A closed post is more "alive"; the only thing that can't be done there is adding new answers. More relevant to this situation, the close state carries the implicit message "please improve this question so that it can be reopened." It doesn't send that "this is permanently bad" signal. And it's relatively easy for a few people to quietly reopen it. For more info, see the network meta's community-generated FAQs on closing, general locking and historical locking.

As for your concerns:

It can only be done via flagging, i.e. community cannot take care of locking without mods.

Locking is a pretty severe action. It should be used only rarely, and in exceptional cases. (I'm using the programmer's definition of "exception" there, meaning somewhere between unusual, non-standard, unexpected and slightly problematic.) We require a mod's intervention to make extra sure that locks are only applied to posts that deserve it. The actual process isn't hard or time-consuming, the mod's role there is really just oversight.

Similarly, it can only be undone via flagging, i.e. community cannot reverse a wrong decision without mods.

For something as disruptive as locking, we didn't want people to get into fights the way we've seen "close/reopen wars" in the past. And as rare as locking is intended to be, un-locking should be a dang unicorn. This is because locking shouldn't have happened in the first place for a post that didn't deserve it or wasn't going to stay locked. So, again, we want mod oversight before a change in lock status is made.

When a thread is locked, it's not possible to edit any answers, post new comments, or even vote on them. I.e. if there is a mistake in one of the answers, it's there forever, and one cannot even comment to point it out. Note that sometimes we are talking about very visible threads with large number of views.

This is an annoying side effect, to be sure. When it comes up, it can be a sign of an error in process. Locking isn't supposed to be anyone's first choice of action when thinking about how to handle a post. In many cases, especially in the case of incorrect information, it may be better to delete a post outright than put a historical lock on it. This is the sort of thing should be considered and discussed before a decision is made to lock a given post.

The displayed message says that the thread is locked "for historical significance", but often there is no particular significance.

Well... don't do that, then. Lock those posts that actually have historical significance, and do something else (close, delete, ignore) with the others.