There was a discussion about it in the A18 dev diary that lasted for a few pages. I don't know absolutely, but basically the number of textures we have available is pretty close to what we had before. Someone posted screenshots of each page from A16 along with a side-by-side comparison to its page in A17.
Essentially what happened is a lot of textures were enhanced. A lot of the colorful choices from before are high in detail now, but dull in color... which gives the impression that a lot are missing. Only a couple are completely gone. Madmole said he would look into getting us some solid color choices in the future.