I think the sad truth is that it is not posted anywhere at the moment. From what I understand, there is work happening now to publish a page describing the different methods and terms of use of the logo on
iiif.io (for instance, it has been suggested that the logo be available as a iiif endpoint, but others can better point to the relevant issues/discussions.
In the meantime, however, I have attached an eps and svg version to this email. These are not necessarily the official final version, but they are the "originals" (by Chris J.) from which most raster versions have been derived.
I hope this helps.