Android 5.0 was a big visual change for Android, but the upcoming M release will make its mark on Android as well. Google is likely going to announce hard cut-offs for Nexus device update support. There will be no more guessing about which devices will get updates and for how long, but that means several older devices are going to stay on Lollipop.
Disclaimer: No matter the confidence level, there's always a chance product updates, features, and some or all details will be changed or cancelled altogether. As with all rumors, nothing is 100% until it's officially announced.
We're rating the update scheme as a 9 on the confidence scale. Our sources on this are solid, so we believe it will be announced at some point before Android M hits devices, but the I/O keynote would be a good time.
We've been told that Google will announce a new update guarantee for Nexus devices alongside Android M. This is different than the failed Android update initiative from a few years ago. Updates on non-Nexus Android devices are still the wild west.
So here's the deal—Nexus devices will get major system updates for two years and security patches for three years from OS release date. Security patches will alternatively be guaranteed for at least 18 months from the date of purchase in the Google Store/Play Store (whichever is longer). These rules apply to past Nexus devices as well as new ones, and that means some phones and tablets are coming to the end of the line. The Nexus 4, Nexus 10, and Nexus 7 2012 will not get Android M, but will still get security updates for a little longer. Only the N7 2012 is at the very end of its rope.
The N4 and N10 came out in November of 2012, so they're well past the two year OS cutoff. The Nexus 4 was out of the Play Store in fall of 2013, so the three year security patch cutoff will be observed for this phone. That means this fall will be the end of N4 support. The Nexus 10 was sold for much longer, only dropping out of the Play Store in fall 2014 (along with the 2013 N7). So in this case it will have security updates through early 2016. The 2012 N7 wasn't nearly as long-lived as those devices. It came out in the summer of 2012 and was dropped about a year later. It's already past the major update limit (I guess Google was feeling nice with Lollipop), but this summer is the end of security patches.
As for newer devices, the 2013 N7 should have system updates until later this summer and security patches through summer 2016. The Nexus 5 will be on the full OTA train until this coming fall, then it should drop to security patches for another year. The N6 and N9 are still for sale, of course, so we don't know for sure when security updates will stop, but they'll have system OTAs until at least fall of 2016.
We don't know for sure if Google will announce the update guidelines at I/O or will wait until Android M (or another Nexus) is released. Still, the stage at Google I/O is a good place to make the update policy clear.