I didn't expect to find a mostly brown color palette so attractive, but the visual design on this game is well done! The music also fits the game, it's exciting without being too overbearing, and I love that each area has it's own theme. In fact, it was the music, not the map, that first made me realize the map was sectioned off into different areas.
The rooms and progression are well designed, and I think each new mechanic is intuitively introduced.
In the opinion, the heart system is unneeded and I think the game would provide enough of a challenge without it. It got annoying being reset back to the beginning every 4 hits and having to run through multiple rooms, just to face the same difficult spot and possibly die again. I think continuing to respawn in the same room would be fine. Ultimately, it was the annoyance of wasting my time that made me stop playing midway through.
I see how the deaths after using the wires ties in with the theme (and the electrocution animation is nicely done), but I think this is also unnecessary. Unlocking a shortcut is nice, but since I often have to return to the area to finish exploring it, being sent back to the beginning still feels bad.
I was also a bit confused by the hearts dropped when I died. While I played, I thought they worked in a similar way as in Hollow Knight, where I had to pick it up in order to regain my full life force. As a result, I went out of my way to collect my dropped heart, even jumping into death to reclaim it. I have since read your comment about what they do, in which case I suggest placing the hearts on a safe floor in the room you die in. For example, if you die on spikes, the heart would be placed on the nearest ground, not on the spikes.
Overall, these criticisms are made as if this was a finished, published game. The visuals and sounds are stunning, and the game design is very clean. The whole game is polished well. It is only a couple design choices that I personally don't like (but perhaps hardcore platformer fans do). The fact that this was done in a 3 day jam is insane. Very nicely done! I want to come back and finish playing it when I'm feeling in more of a platformer mood.