As far as I'm aware they haven't announced anything for compatibility. Though I wouldn't be surprised, generally five years after manufacturing on a model ends it becomes "vintage" in Apple's repair systems in most of the world, and is no longer supported.
"According to Apple, the official compatible hardware list of Macs capable of running Mac OS Sierra 10.12 is as follows:
MacBook Pro (2010 and later)
MacBook Air (2010 and later)
Mac Mini (2010 and later)
Mac Pro (2010 and later)
MacBook (Late 2009 and later)
iMac (Late 2009 and later)"
Edit: My original post was wrong and I found this.
"Sierra is the first version of macOS since OS X Mountain Lion, released in 2012, that does not run on all computers that the previous version supported.[8] Developers have created workarounds to install macOS Sierra on some Mac computers that are no longer officially supported.[9]"
Here is El Cpitan support:
"These computers can run El Capitan, provided they have at least 2GB of RAM:[9]
iMac: Mid 2007 or newer[10]
MacBook Aluminum: Late 2008
MacBook White/Black: Early 2009 or newer
MacBook Retina: All
MacBook Air: Late 2008 or newer
MacBook Pro 13-inch: Mid 2009 or newer
MacBook Pro 15-inch: Mid 2007 or newer
MacBook Pro 17-inch: Late 2007 or newer[7]
Mac Mini: Early 2009 or newer
Mac Pro: Early 2008 or newer
Xserve: Early 2009"
So maybe the Sierra supported computers list will remain the same for the next few big yearly macOS updates?
Also maybe you can hope for developer work arounds to let you install the latest MacOS for a few years even if they stop supporting it?