Oh, no one knows either way about Android M support right now. I've seen lots of speculation and people talking about device strings but none of it seems concrete to me. I just meant that so far my Nexus 4 is on the same version of Android as my Nexus 6, and with some luck it will continue to be supported through M. (It's already long past Google's 18-month or so support window)
So, even the vaunted support for the Nexus brand is only "Guaranteed" for a year and a half?!?
FFS, Apple is still supporting (even up through the current version, iOS 9) my iPad 2, which was first sold on March 11, 2011, over FOUR years ago (a millenia in mobile-device-years). Apple has even released versions of iOS specifically targeted at improving performance on the iPad 2.
Similarly, Apple also still supports (even up through the current version, iOS 9) my iPhone 4s, which was released on October 4, 2013, the day before Jobs' death. In fact, I often thought that the real, "secret" reason behind the model name "4s" was "for Steve", "3GS", etc. notwithstanding.
And I believe there was even a relatively-recent "Security Update" for the iPhone 3GS, which was introduced on June 8, 2009. Support ended for the 3GS on or around September 12, 2012.
As far as standalone Security Updates, in May, 2011, Apple patched versions of iOS back to iOS 3.0 with their iOS Update 5. Quite frankly, I don't understand that Security Update, especially considering there is an Apple document dated April, 2015 that talks about it.