As long as there exists a bijection between whatever you choose as an index and the natural numbers starting from 0 it is fine. (I.e. the range of valid indices must be a countable set) In your example that bijection could be:

  3 -> 0
  4 -> 1
  5 -> 2
  6 -> 3

This works for vectors as well, so why not have a range from (0,0) to (5,5) to index into an array arr? You could write the function that does the mapping manually:

  arr[(x,y)] = backing_array[x / 5 + y] //bounds checks omitted

But here it can be automated quite simply to allow for vectors of even 3 or 4 dimensions.

Just know that custom indexes / ranges are not automagically broken. Personally, I like how much easier it is to read the intent with custom indices.