Okay, that idea I had about writing a plugin to add a “Retired” status for Movable Type entries? Not going to happen. At least not any time soon. The post status types are pretty well hard-baked into the core MT code, and adding a new status would require some hacking around in places like Entry.pm as well as additional plugin code. Modifying core files means re-modifying them every time you install a new version, and that’s just a pain.
(However, while digging around in the posting status area of the MT core, I found a bunch of dormant code relating to scheduled posting of entries. There is a posting status code of “Future” defined right next to “Draft” and “Publish”. On the other hand, there has been a status code of “Review” in there for some time, too, and it hasn’t appeared in the UI yet, either. It’s probably not worth getting excited about scheduled postings coming to MT in the next version.)
In the absence of a simple plugin, here is a relatively simple alternative way for “retiring” outdated entries in Movable Type.