There's actual legal precedent that stages with hitboxes ought to be banned from competitive play. At least in the United States, video game tournaments lie under a discretionary gray area as a game of skill, but if there are man vs. computer elements in the game as part of the interaction, it no longer is considered a game of skill and is kin to gambling (to account for video poker).
That being the case, hitboxes are how the score is altered in the game we play. They deal knockback and percent with the win criteria is eliminate your opponent's stocks or have more stocks then less percent. There's a logical distinction that one can make between manipulating the smashville platform for tactical gain (which lies soley in player vs. player because the smashville platform does not alter the score of the game alone) compared to the Halberd claw targetting a player and hitting them (which can exist outside of the context of player vs. player) since the stage's hitbox is interacting with a players hurtbox which happens independent of an opponent's hitbox and by doing damage or knockback alters the score in the win/loss criteria. For example, if a stage element had a hitbox that required an opponent to interact with via their hitbox (like let's say hypothetically Green Hill Zone's Course Flags existed on smashville) they would be acceptable as it requires manipulation of a player. However, randomized stage elements without hitboxes requires a player to capitalize on the element to punish their opponent, which is by definition player vs. player (like pokemon stadium's non set phases or Delfino's set track).
But as far as hitboxes on stages go, most people apply the "reasonably avoided" argument. The criteria of "reasonably reactable" is incoherent if using the discretion criteria provided by Sirlin. Is reactable to a hazard 12 frames average human reaction time? Is it the total animation length of the longest move in the game? Is it that longest move animation plus average human reaction time? Is it average human reaction time, plus the longest animation length, plus another factor of human reaction time to account for reacting to when the hazard comes out AND if it will hit you? Picking one is pretty arbitrary so it's better to remove the stages with hitboxes in the first place.
With that criteria in mind, it's why the KC stagelist is: Battlefield, Smashville, Lylat Cruise, Yoshi's Island Brawl, Final Destination, Castle Siege, Pokemon Stadium, Delfino Plaza, and Rainbow Cruise (and sometimes Frigate Orpheon and Pokemon Stadium2) since the stages have no hitboxes and would fall under player vs. player in manipulating terrain for tactical advantage.
