Besides unveiling the Nexus 5X, the Nexus 6P, and the Chromecast 2, Google also announced that it will start rolling out Android 6.0 Marshmallow for the Nexus 5, Nexus 6, Nexus 7, Nexus 9, and Nexus Player from next week.
The company noted that it will not be updating the Nexus 4, Nexus 7 (2012), and the Nexus 10 to Marshmallow. Considering that all the three devices were released in 2012 with some version of Jelly Bean, it does make sense for Google to not update them to the latest version of Android. While the Nexus 4 and Nexus 10 do have ample power to run Android 6.0 Marshmallow, it is likely that Google decided against updating them due to the sheer engineering effort required.
#AndroidMarshmallow will begin rolling out to Nexus 5, 6, 7, 9 and Nexus Player starting next week.
— Android (@Android) September 29, 2015
Nonetheless, if you own a Nexus 4 or Nexus 10, don’t be disappointed. I am pretty sure it will just be a matter of few days before a working Android 6.0 Marshmallow custom ROM pops up for these devices. The Nexus 7 (2012), on the other hand, was already unusable on Lollipop, so I doubt its owners are going to complain about the lack of Marshmallow update for it.