This category includes computer software that is no longer maintained, like:
Software that has been officially discontinued by the original developer and is not maintained by any other third party (like some kind of community)
The company that used to maintain it went bankrupt or ceased to exist for a variety of reasons, and no other company is going to maintain it (even it it was not officially discontinued)
Even if not (yet) officially "unmaintained" or "discontinued", software that has lots of serious unpatched bugs for a long time, especially if they are security vulnerabilities - open to debate which software to include based on this (Secunia might provide useful information regarding this)