Version 1.9 only changes the mod interface for extensions. It is now possible to add fences with random poles or middle segments.
To use this feature, the fence config needs to be adjusted like following:

snowball_fence_wood = {
  {
    post = { -- list instead of single reference string
      "snowball/fences/new/wood_post.mdl",
      "snowball/fences/new/cow_post.mdl",
    },
    middle = { -- list instead of single reference string
      "snowball/fences/new/wood_middle.mdl",
      "snowball/fences/new/wire_wood.mdl",
      "snowball/fences/new/cow_middle.mdl",
      "snowball/fences/new/stockade_middle.mdl",
    },
    length = 3.0,
    offset = {0.15,.5,0.15},
    invertedModel = true,
  },
  ...
}

Additionally, the .con file for this fence type needs to be adjusted:

updateFn = function(params)
  if params.snowball_fences_type == 0 then
    return fences.previewAdvanced(
      params, 
      fences.fences.snowball_fence_wood[1].offset, 
      fences.fences.snowball_fence_wood[1].post[1]) -- added index [1] to select the model from the list.          

  else
    ...
  end
end