I think that if you're going to use the tag, then the recommended way of doing it should be the way Christian Foellmann did it: by also adding the tag to the short description as well as to the first paragraph of the long description.

By just adding the tag like Pippin has done, it will only show for people explicitly looking for the tag.

Correct me if I'm wrong, but adding the tag basically means that the developer will no longer update the plugin, right? So, if a plugin author does not add any other information, users unaware of the tags will still install the plugin, with all risks involved.

I would therefore suggest tag, short description, long description; that is if you really want someone to take over development...