I don't think they should ban stages at all, if you're good enough to play in a tournament you should also be able to play any stage with your main efficiently.
I don't think people can't play on certain stages. It's just obvious some characters have bigger advantages on certain stages than others. Banning 1 stage each shouldn't be that much of an issue.
I like to play my Peach on Battlefield becausd Turnips + Her Decent Juggling Ability allows me to continue my combos easier.
Plus with its low ceiling it allows for her Fsmash Frying Pan to kill at pretty low percentages. On the Top Platform.
As well as her Up B.
Same goes for Mario's Up B on the top Platform.
It also allows certain characters to have more area to recover than others instead of being caught out or read easily. Such as Diddy recoverying or DK, and giving people a better chance for both sides instead of being super one sided.
Banning stages doesn't really show you're not good enough. But if you're doing anything to win. What's stopping a Shiek Main from picking Battlefield every game and just having open season on your ass for 2 games?
I say banning 1 stage shouldn't be an issue. 2 is a bit much due to the small stage selection to begin with.