Yes, OGC WMTS has been implemented
Last week I attended a meeting where discussions on WMTS broke out. During the verbal sparring, some folks made the comment that "nobody has implemented OGC WMTS!" Well, not true - proof positive above where multiple REST WMTS from CubeWerx and others are shown "in action" with Gaia.
The episode reminded me of several years ago when established vendors decried WMS indicating, "but nobody has implemented OGC WMS!" I guess the more things change, the more they stay the same ;-)
With the release of a proposed standard for tile-based web mapping, Web Map Tile Service (WMTS), the OGC is poised to provide an open alternative to proprietary web mapping services like Google Maps. Worked on quietly by the gurus at CubeWerx, CREAF and the Autonomous University of Barcelona - the candidate WMTS Interface Standard is much like OGC's popular WMS, but with faster performance.