So while every model has a date, the vast majority of them aren’t “official”, meaning support could extend beyond those dates. So far, the only two devices to have been “Officially announced” are the CR-48 (December 2015), and the Samsung Series 5 (June 2016), and remember the CR-48 is still receiving updates. So even official doesn’t mean official.
There would be three reasons why a Chromebook couldn’t run the latest: speed, memory, or technical limitation. Since Chromebooks haven’t really changed since their inception, this is good that Google isn’t standing firm on the no updates. We have a bunch of Samsung Chromebooks that will reach EOL this summer, but they still run fine.