I'll begin by stating that I don't believe we should use tags for book-keeping purposes. Tags are intended to help categorize what people are looking for to find the questions that have their answers. The problem with book-keeping tags is that unlike the categorical tags we use for identifying questions, these are only meant to last until the problem is solved. Tags are traditionally permanent and changes to tags are typically done because of incorrect tagging as opposed to a changing status of the question. This is a similar reason as to why [upcoming-games] is not a good tag, since what constitutes an upcoming game changes as games become released or advertised. Another issue is that adding a book-keeping tag typically doubles the necessary number of revisions.
The better course of action for book-keeping is to alert the appropriate people. In the scenario of out-of-date information, you want to alert the people with the out-of-date information - the post authors. Fastest and cleanest way is a comment, which won't bump the question but will still alert the author. If you're in chat or otherwise in communication with other users, also consider requesting comment upvotes in order to ensure that it is immediately visible.
As mentioned by badp's answer, you can also opt to edit in the fact the information is out of date. This bumps the question, which means that new eyes can see it, recognize the out-of-date information, and possibly provide updates themselves.
Which highlights the remaining course of action, which is skipping the book-keeping and just administering the corrections yourself. If you know the new information yourself, you needn't always have to wait for the existing content to be updated. The Necromancer badge doesn't just reward people who target questions which have decayed in obscurity, but also rewards people who provide a more up-to-date answer to an old and/or formerly solved question. Of course, providing up-to-date answers is mostly applicable when the question still makes sense post-update, which is also noted by badp.