The Apple Watch Series 3 will be oldest device supported, although given the trend, watchOS 7 could also very well be the last update for the Series 3, as next year we imagine that watchOS 8 could drop support for the Series 3 model, but that’s just speculation on our part. That being said, it seems that watchOS updates seem to be shorter compared to iOS.
It also brings up a conundrum where people generally don’t upgrade their watches very often, so the idea of walking around with an obsolete piece of tech strapped to your arm doesn’t sound very appealing. However, we should note that just because the older models will not receive watchOS 7 doesn’t mean that they are useless.
They can still very much do what they intend to do, but will not have access to the improvements and new features of watchOS 7, such as a built-in sleep tracker and automatic hand-washing detection, just to name a few.