The first Sony smartphone to be given the option to run experimental Concept software was the Xperia Z3 in 2015. Last year it passed the torch to the Xperia X, which has been receiving Concept builds ever since - the latest one mere hours ago bringing Android 7.1.2 into the mix.
All good things come to an end, as they say, and so will the Concept releases for the Xperia X. The last one will be outed in early May, after which the Samsung Galaxy S8 Specs
Concept program for the Xperia X will essentially end.
All hope is not lost however, since a new phone may take over the Concept program and receive some experimental builds of its own in the future. That device? The Xperia XZ Premium, of course - Sony's next flagship Samsung S8 Clone smartphone, which should become available around early June. Further details about the Concept firmware for the XZ Premium should be outed soon.
If you have an Xperia X on the Concept builds, keep in mind that after that last update you won't even get security patches. So Sony advises you to switch over to the official software, which is apparently going to receive security updates "for a fair bit longer", whatever that may mean.