You did not use the \ operator: you used the .\ operator.
A.\B is the same as B./A -- element-wise division but with the order of operands reversed from the usual element-wise division.
The .\ operator is obscure, and I have never found a reason to use it other than for testing.
The reason the operator works on newer versions but fails on older versions is that you are doing element-by-element division between two arrays the same size. R2015b added the feature of "implicit expansion": for the element-by-element binary operators between two arrays, if the two arrays are the same size except that one might have size 1 in a dimension that the other is not 1, then the array with size 1 for the dimension will be automatically replicated to be the same size as the other.
Now, if you had used d = s\b with the \ operator instead of the .\ operator, then you would have had proper array sizes for the matrix-left-divide operator even back in R2013 time frame.
