Stage legality

In competitive play, a banned stage is a stage that is not eligible for tournament play. A stage may be banned for a variety of reasons, such as:
 * providing extreme advantages or disadvantages for certain characters
 * having a bad layout that encourages degenerate strategies like camping
 * being too large or too small, increasing or decreasing character survivability beyond reason
 * having stage hazards that disrupt normal gameplay
 * possessing exploitable glitches that break the game when abused
 * having many graphical elements that increase the amount of lag to unreasonable levels
 * having stage elements that remove an important aspect of the game, such as walk-off edges

Basically, any stage that proves to be detrimental to the competitive nature of the game is typically banned.

List of banned stages in Super Smash Flash
This is the current ban list for Super Smash Flash, as of May 23, 2015.

List of banned stages in Super Smash Flash 2
This is the current list of banned stages for Super Smash Flash 2 v0.9b, as of September 24th, 2016. There are three levels of stage bans: Note that this banlist only covers singles, as the competitive doubles metagame is virtually nonexistent at this time. It should be noted that all these stages are banned regardless of the hazard switch, unless stated otherwise.
 * Banned: Stages that are banned in all forms of competitive play.
 * Banned in backroom list: Stages that are banned from the official Backroom stage list, but are sometimes legal in experimental rulesets.
 * Banned in backroom online list: Stages that are banned from the official online Backroom stage list, but are legal in offline tournaments and experimental online rulesets.