This is according to a listing on the Chromecast support page where all the newer Chromecast models have the firmware version 1.42.168007 while the first-gen Chromecast is running on the firmware version 1.36.159268. Google has yet to officially confirm if they are ending support for the device, but in a statement made to 9to5Google, they note that they will continue supporting it in terms of bug and security fixes.
That being said, it shouldn’t be surprising that Google isn’t focusing their efforts on the first-gen Chromecast anymore. This is because the device was first launched back in 2013, a good 6 years ago, so for Google to have supported it for as long as they have is already pretty good. However, if all you need is basic Chromecast functionality, then we suppose not having major updates isn’t that big of a deal, although given that Chromecast devices aren’t exactly expensive, an upgrade could be worth looking into.