Projects that were announced but never released. Like most volunteer endeavors, Myth II has had it's share of ambitious, greatly anticipated projects that were announced by their creators but which were never finished. Some ended up released as betas or in really rough form, some were apparently never even started.
If you want to see a list of Vaporware projects, just do a title search using 'vapor'. In the graveyard, the only things classified as 'vaporware' are projects that were never released, not just "Beta" versions of maps. Although some vaporware products were never officially released some were released later in Beta somewhat playable form. A good example of this kind of beta-vaporware is "The Great War" - a campaign that has several playable levels.
Also, beta versions of maps aren't classified as 'vaporware' because some betas are actually finished fully-playable maps or plugins which the mapmaker just never got around to renaming/renumbering or polished.