Just figure I'd give my two cents about stage legality and whatnot, since there's a lot of hot debate about it as of right now.
What makes a stage banned isn't it's disruptiveness; it's its potential for hazardous disruption and making the outcome of the match more based on variables outside the players' control.
Take stages like Yoshi's Island (both Brawl and Melee apply here) and Fountain of Dreams. These stages are, without a doubt, disruptive. The supports on both Yoshi stages can save players, and the rising and falling platforms on Fountain of Dreams creates unfavorable situations in the neutral for players that just so happen to be near them when they rise and fall. But, these things all are non-hazardous. None of these will directly kill you; they will just put you into a position that you may either like or not like. At the end of the day, it is still your responsibility to get out of the situation carefully and your opponent's to try to capitalize on it; i.e., what you're trying to whenever you're in a bad situation anywhere on the map, hazard or not. Even if certain stage aspects create for situations where a person may be unfairly saved (the Yoshis), it is still the player's responsibility to react to it, and he/she is completely able to do so. This type of hazard does not tamper with the player's ability to out-compete his/her opponent.
However, the other types of stage hazards that
do disrupt a player's ability to compete are the ones that should be banned. Take, for instance, since it's being oh-so-talked about in this thread, Corneria in Sm4sh. NOT EVEN including the new glitches found out by the community, the hazards on this stage put who ever gets affected by them in a position that contains an outcome that operates OUTSIDE of skill level of each player. Player X knocks Player Y into the air. Player Y jumps and is hit by the drive-by hazard. No matter what Player Y does, he has the potential to be killed by that lazer, and is in every situation, is going to be damaged by it. Not because of a skill difference, nor because of being out-competed; he just happened to be in the wrong place at the wrong time.
"But shouldn't it be Player Y's responsibility to NOT be in the wrong place at the wrong time?", you may be asking to yourself. And in regards to player vs. player combat, then yes, he does. But it is safe to assume that at the end of the day, the player who can objectively be considered better than someone else is the person who managed to, through out-competing them and being more skillful in every facet of the game, beat them. With this being the overall guidelines to the best in the competitive system that Smashers have constructed, stages should be chosen that reflect this theme. If a person wins on Corneria, one could make the arguement that it is not because they managed to out-play their opponent; rather, it is just because they managed to "out-environmented" their opponent. This rationale is already reflected in almost every rule in the system; Smash Balls are banned so that the winner is determined by who can take off 2 or 3 stocks the quickest, not by who can get the Smash Ball the fastest.
To me, THIS is how we should be approaching which stages are and aren't legal in Sm4sh.
tl;dr, It isn't disruptions that lead to being banned, it's hazardous disruptions that lead to being banned. It also having things that take away from the overal competitiveness of the scene that lead to being banned.
also all of this may be wrong because i'm fairly new but ayyy why not shoot it out there and see what happens