It's also worth noting that the reason stages used to be legal but aren't now is because it took time to realize what is good or bad in a stage. Brinstar lava is more punishing for characters with poor mobility than good mobility. In a match played on Battlefield, for example, Jigglypuff gets Ganondorf offstage when Ganondorf is at 20. He takes a couple nairs, but wisely saves his jump and recovers. Next game, same scenario, but because Ganondorf has worse recovery and mobility options, he gets hit by lava as he tries to recover low. And now Puff can just keep him in lava hell and rack up way more damage than if game 2 had just been played on FD.
And well, some stages/aspects of stages have to be banned to make things fair for all characters. Didn't Duck Hunt get banned eventually in Smash 4 when it was realized that Mac couldn't reach the tree with his double jump? If not, it should have. It may seem annoying to ban a stage just because of one character, but if you don't, now everyone who mains Mac is in for a bad time. Walls can provide disproportionate benefit to characters that have spammable down tilts, being able to infinite or near-infinite just by using a single move. Ceilings make it easy to trap and infinite/near-infinite big characters with projectiles or up tilts. Excessively large stages make it easy for fast characters to get a lead and run away. Walkoffs remove the weakness of poor recoveries that helps balance those kinds of characters, and rewards strong kill moves above all else. I don't think it would be difficult to imagine why stages like these would suck the fun out of the game if someone's character does poorly on them.
Additionally, stages were sometimes legal in the past simply for a lack of options. PS1 is a pretty bad stage in Melee imo, but is legal because there simply aren't any alternatives. This has, naturally, improved as time has gone on. Now in Ultimate, it feels like we no longer need to have questionable stages legal because we finally have enough options.
Is our current stagelist perfect? Probably not. I would have said no, since it used to be comprised of mostly large stages that benefit campers (though even then, I think it was the best we could have done). But now that we have Small Battlefield, I am more optimistic about our stagelist. It's just that we haven't had offline tournaments, so it's hard to judge. But at the current moment, we are at least very close to a stagelist that is well-balanced for all characters