Speaking to Gizmodo, a Netflix spokesperson said the issue was that these older devices run Windows Media DRM, a digital rights management service which has since been replaced by Microsoft PlayReady. Netflix has used PlayReady since 2010, as it is easier to get content from movie studios and other providers in that format. From December 2nd, these older devices which aren't able to upgrade to PlayReady will be locked out.
The devices affected include the Roku SD (N1050), Roku HD-XR (N1101), Roku HD (N1100, 2000C) and Roku XD (2050X, 2050N, 2100X, 2100N), in addition to certain Samsung Smart TV models from 2010 and 2011 and some Vizio Smart TVs with VIZIO Internet Apps. Most of these devices are coming up to a decade old, so it's inevitable that software support for them would eventually end.
Owners of these particular models have had problems with other services as well. Last year, Hulu announced it would end support for both VIZIO Smart TVs with VIZIO Internet Apps and older Roku models. Older Roku devices can still access videos from the Hulu streaming library, but cannot access features like premium add-ons or live TV.