Brinstar is a stage that debuted and has been in every Smash Bros. game since Melee. It seems to be based off of Smash 64's Planet Zebes because both feature acid rising and falling. This one has a bunch of other stuff like the ability to change the positions of the platforms. It also removed Ridley from the background so that he can be fucking playable.
It has a brain in the background, therefore it is and should have been competitively viable, but still became the last stage to be banned until like Yoshi's Story or something has a reason to be banned.