From the Super Mario Wiki
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, games are complete or nearly complete when beta testing takes place, hence most beta elements are scrapped long before this period. However, in case the beta elements are still there, these get changed based on the testers' experience. The best change remains in the final game.
Beta elements should not be confused with vaporware, entire games which never see release.
The full list of beta elements can be found here.