Sigh... where is this idea even coming from?
Alpha demo stage top level is not a square, has multiple danger zones and changes shape/height based on what you trigger. Bottom area isn't even an unintelligible shape due to the way the wreckage fucks with things. It's more like a circle thats been stabbed a few times and left for dead
Circus stage isn't a square either. It's a really, really long oval if anything, and the protruding danger zones make it an imperfect oval. The tiger tier is a circle with an X of death going through the middle you have to avoid.
Flow actually does start out as a square, by the end of the river a chunk is bitten out of it and its height teeters back and fourth depending on where the weight on the raft is. The lower level is also not an intelligible shape, tons of crap in that area.
Sakura is neither a square or a rectangle due to its imperfections, and qualifies more just as an open area level.
Oil Rig is made up of small imperfect, destructible tiers that come off as narrow platforms you have to fight on until you get to the bottom. Small by design to get you to the bottom area fast, but takes a hit or two on each surface before it breaks so it feels more like you didn't just guess wrong randomly. The bottom area (that we know about) is cross shaped if anything, and judging by the screenshots I wouldn't be surprised if there was more to this level than we were aware of.
Sweat -- You got me. It's a boxing ring.
Hotzone -- If you were going to call this a rectangle, you'd have to call it two rectangles.... but it feels more like you're fighting in a long hallway with gunfire. You can move forward, but you can't move back.
Unnamed Tag Stage -- large open area with electrified fencing around the edges. Looked Oval Shaped.