Answers sometimes become obsolete. New solutions arise that were not technically feasible at the time that the question was originally posed. People decide that the old way of doing things is no longer a good idea. This goes for answers on main sites as well as on meta sites.
In How to deal with obsolete answers?, several suggestions were raised, but I don't find them satisfactory.
- Deleting is not appropriate, I think, since it removes all trace of the answer, and even obsolete information is sometimes useful. It leaves a void, and there is even a risk that the void could be filled by someone else writing a similarly obsolete answer. I also believe that is is inappropriate to strip the original poster of reputation points that were rightfully earned in the past.
- Editing the answer to provide a new solution is an underhanded approach, since the practice of radically altering posts is prone to abuse. Who is to say that all the voters who upvoted the original answer agree with whatever new advice is added? It's even worse if someone other than the original author of the answer violates the author's intent. Editing also fails to confer reputation commensurate with the effort, if the new answer is completely different. Additionally, it might not be desirable to bump an old post onto the front page.
- Downvoting To counteract all of the upvotes, you would have to rally a downvote brigade, which is neither realistic nor desirable. There's no need to hurt the original poster's reputation just because an answer is no longer relevant. Furthermore, all previous upvotes are locked in unless the post is edited, so the concerns raised above about editing also apply.
- Commenting is insufficient, because it could be buried at the bottom of a long thread. Also, comments on Stack Exchange are designed to be of a somewhat transient nature. Comments also don't alter the order of presentation of answers.
- Locking is currently possible only for a "Content dispute" or for "Offtopic [sic] comments". As the guidelines say, "posts should only be locked when something seriously bad is happening. A locked post cannot be changed in any way or voted on". I don't think that obsolete information qualifies as "seriously bad", or that editing/voting should be prevented.
I think a feature enhancement is called for!