While OEMs put an end to updates after a year or so the development community continues to release updates for phones long after they have been end of lifed. I can remember last year a Lollipop Rom came out for the Droid X. If the phone were not basically extinct at this point I am sure someone would have a Marshmallow build for the device.
Several version of the Galaxy S5 including Sprint, US Cellular, and Vodafone are now officially supported with CM13 Nightly builds. Added to the list are the AT&T LG G3 and Xperia SP. This brings Marshmallow to devices that otherwise would never see such an update.
via Cyanogen