This is superb work.
I've seen quite a few people have a go at this kind of 2D/3D mechanic over the last few years but I've never seen it implemented cleanly before. The first level where you have to engage with the 3D is great - super well timed "oh shit" moment when you realise the staircase moves diagonally.
The gravity mechanic with the walls is also cleanly done, although I would probably have made that toggleable by the player. I can see a lot of potential puzzles and platforming gauntlets that would require the player to move in 3D and rotate the level whilst moving.
I would say ignore everyone that said a tutorial is necessary here. I think with this kind of game the joy is in the discovery. The fact it's minimalistic is part of it's charm. With more levels you could compound on the ways the two mechanics interact with each other and have a lot more epiphany moments. Telling the player how to solve the puzzle would ruin it, I think.
Visuals are very nice. Clean, bold, and readable - perfect for a jam entry.
The main point of criticism for me is the player controller. Feels a bit floaty. If the jump curve was a bit tighter and heavier and the character model did the whole "stretch and compress" thing it would feel a lot jucier. Can't mark you down for it though as this game has so much going on mechanically already :D
I know it would possibly make the 3D less interesting, but perhaps it would be better if the 3D camera angle wasn't controllable. Maybe you hit shift and them camera spins automatically to show an isometric angle in the direction the character is facing.
Definitely one of the best entries I've played this jam.