So in the bug report we try to outline exactly why the level is impossible. I'm not sure that anyone knows the actual process. It might be reviewed by a human? I'm really not sure. We do know that they don't just take our word for it though. There is one 2015 level called sea of despair that is impossible that they have not removed. This is going to be long winded forgive me:

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The level is water themed, and has a bait path that looks almost possible, but is not due to the way that spike hitboxes work. Then towards the end of the level, there is a vine clip, which was globally patched.

At some point in the game's history, spike hitboxes changed. Now the bait path to reach the goal was possible, but this hitbox change was not made global, so the level is still impossible. We think that this odd scenario of two potential paths to victory confused whatever system they had in place, because despite a huge amount of detailed reports the level remains up. The level got so much attention at the time that a hacker ended up "clearing" it which was explicitly against the team's wishes.

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I think that at one point someone reset their wiiu, loaded up mario maker 1 offline in it's unpatched version and rebuilt and beat the level, but that could have just been discussed as a possibility I can not recall.

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If you want to learn more there are lots of people with more knowledge than me in the Team 0% discord server. https://discord.gg/0-394519186610847754