As quoted by Xperia Blog there were various rumors that said Xperia Z5 is the last of the Xperia Z series. As soon as announcement of the Xperia X series there were many reports suggesting there will be no Xperia Z6. So Xperia Blog has asked for an official comment from Sony on the matter. They've got one. But the sad news is Xperia Z series is no more. So here after there Sony will continue their flagship series with the Xperia X series.
“The Xperia Z line has reached its culmination – Xperia X series represents a new chapter and evolution of our product strategy. Whilst the Xperia Z series was all about bringing the best of Sony’s advanced technologies to smartphones, Xperia X series is based on bringing smart, adaptive Sony user experiences across camera, battery performance and hardware / software design.”Via 1 2