The New 3DS also did eye tracking, which is why I said it did.

The original 3DS didn't but the "New" one did eye tracking like this, and for both eyes since it had a 3D screen.

I bought a new 3ds and it was absolute bliss. The mario bros 3d game was incredibly intuitive to play, that 3ds has spoiled any 3d platformer for me. I tried playing the new 3d game on the switch and even though it has shadows, I kept missing platforms etc.

I think the original 3ds's lack of tracking is what turned it off for most people, though.

No, it relied on the fact that the player would ~always be looking straight at the screen at a relatively fixed distance. There was a slider you could use to set the strength of the depth effect, all the way to disabling it completely.

It was fun and worked pretty well, but it was very much a gimmick.

EDIT: Just saw the other comment, I didn't know the New 3DS had actual eye tracking. Neat!