20xx is a modpack which isn't allowed in most tournament scenes. If it were legal then TOs would have to spend valuable time and resources to verify the legality of each modded game. 20xx Yoshi's, Dreamland, and FoD are really good stages, but TOs will not risk letting in hacked copies of melee that may lead to cheating.
And I agree Pokemon Stadium is jank for allowing for infinite combos, but I don't see this as a reason to ban it. I mean it's no different than wobbling, except wobbling is infinite and can happen on any stage, while infinite shine combos can only happen on Pokemon Stadium and for limited amount of times. Instead of banning Pokemon Stadium due to these combos, I suggest the players adapt their playstyle to avoid them, much like how the metagame had to adapt to the emergence of wobbling.
Severe transformations are those that interrupt gameplay and hurt or benefits the players. An example is Rainbow Cruise, the players must constantly be on the move and have little time to actually fight each other. Then the game no longer becomes player-vs-player, but rather player-vs-environment. The point of a tournament is to see how well can a player outplay their opponent, rather than "how well can a player use their environment" or "how well can a player avoid obstacles."