Yoshi's is basically Battlefield but smaller and with slopes. I'll still call for Spring Stadium to be legal dammit.
The whole point of counter picking stages is to potentially give yourself an edge and force your opponent to switch to a secondary or adapt. Like for example: Bowser thrives on Battlefield because his grab/slam can get early kills off the top platform, so taking him out of that removes an option and puts him at a disadvantage
So why shouldn't Spring Stadium be legal? Two smaller ceilings near the edge gives Mario less of a chance to ladder you off the top, a potential counter to Luigi's chain grab, Up+B if he does it in the wrong spot!
Sure, I'll play devil's advocate then.
Ceilings are an aspect of the stage that is completely unique to that stage. For every other aspect, we currently have at least 1 other stage (with 1 exception) of that aspect being on other stages.
Slants? Yoshi's Story and Lylat.
Walls? Kalos and Yoshi's Story.
Smaller blastzones? Yoshi's Story and Smashville.
Smaller stage (10 units or so)? Yoshi's Story and Smashville.
The only stage that has a unique aspect that other stages don't is Town with its moving platforms. However, the smash community has made it clear that moving platforms are viable, and how much they would love for FoD with hazards on to be legal when it eventually gets fixed.
So, if Spring Stadium has a completely unique feature on it, it poses the question: Is that okay?
Here's why the answer is no: During the stage selection process, a specific number of bans and rules are applied between the players playing a set. The purpose of these rules is to eventually allow counterpicks which exist grant an advantage to the losing player. But with current lists, simply putting Spring Stadium on there would skew the lists in favor of characters who can abuse the ceilings more (most notably characters with steep angled aerials and similar features). This isn't assuming that current lists are perfectly balanced, and that adding Spring Stadium would unbalance it, though. This is assuming that those types of characters already have the proper advantage across the stage list.
For example, if you benefit off of smaller vertical blast zones, you're probably going to dislike how Spring Stadium can lose you a game by saving your opponent. The opposite applies as well to horizontal blast zones.
So, what gives? Does it imbalance it too much? Well, that part is extremely subjective of course, but the idea that you may have one additional stage you need to ban because you're character simply has a harder time killing on it or the opponent's character has a far easier time killing on it, poses the question: Does it matter?
I think yes, then. It does imbalance it. By having that unique aspect that no other stage has, it genuinely does imbalance the list because you can't simply give an extra ban and hope for the best, because the common list as is already is skewed towards camping on the large stages, meaning there are less small and multi-platform stages to go to. What if one of the campy characters ends up being really good with those ceilings? Well, then it's a problem.
Devil's advocate over. This isn't to say it isn't worth testing, of course not. I think the stage is actually super fun, but I do think that it having such a unique aspect from any other stage can become an issue with the counterpicking process.