Found on the developper's website:
The image mosaic plug-in for the GIMP is no longer supported or distributed. Mr. Robert Silvers, the holder of a patent related to the technology that was used in the plug-in, argued that the software would directly infringe his patent rights. It is not clear if the patent is applicable in this case. But I have neither the time, interest or money for legal action. So I complied with the cease and desist request.
PHOTOMOSAIC® is a registered trademark of Runaway Technology, Inc. The photomosaic process is patented (US Patent No. 6,137,498) and protected by the patent, copyright, and other intellectual property laws of the United States and other major countries.
It seems though that someone else has taken the development further:
This software is currently being updated for Gimp 2.2. Please check back regularly. In the meantime, the Gimp 1.x version and instructions can be found below, although they may be somewhat unreliable.
This second version of the plugin avoided the technology covered by the patent or ... sanely ... ignored it?