From the Super Mario Wiki
(Redirected from Beta)
Beta elements get their name from the period at the end of a game's development known as Beta Testing, when the nearly complete build of the game is debugged and playtested by a small group of people. Despite this, most games are complete or nearly complete when beta testing takes place, so most beta elements are scrapped long before this period.
Beta elements should not be confused with vaporware, entire games which never see release.
The full list of beta elements can be found here.