How would you have described it?

Your description of slice rotations is really bad

M(idle) the layer between L and R, turn direction as L

E(quator) the layer between D and U, turn direction as D

S(tanding) the layer between F and B, turn direction as F

For the layer between L and R, what is turning in the direction of L?? That makes no sense, honestly just made it confusing.
Now I am confused for the other slice rotations, Like for E, does the slice rotate to the left or right, and for S, does it rotate clockwise or anticlockwise?

I agree. Magic numbers in code are always confusing. b'a' + 1 gives more context.

Despitee terrible efficiency, funny enough its still O(n). :)
O(25n)==O(n)

Truly beautiful

You dont need to match 0 here. THe if condition covers that case already

I like the use of expect instead of unwrap. Its the more "correct" way. Even though irrelevant for that particular kata

Using a hash set would be more efficient instead of vec

Haskell translation

I can't wait to solve new ones, thanks.

I believe you are correct and the author made an off-by-one error there. Fixed.

In the description example when the author says the input coordinate is (2,2,2) is that 1-indexed? So the chosen cell is the one with value 13? Because the example answers make sense to me if 13 is the center cell but if 26 is the center then I'm quite baffled and will have to think a bit more about this problem :D

fixed

fixed