Apple today confirmed that watchOS 27 will not support the Apple Watch Series 8, Apple Watch Ultra (first generation), or Apple Watch SE (second generation), effectively drawing a line at devices equipped with the S9 or S10 chip.
The only Apple Watch models compatible with watchOS 27 are the Apple Watch Series 9, Series 10, Series 11, Ultra 2, Ultra 3, and SE 3.
The cuts are the biggest loss of latest generation software support for the Apple Watch to date. watchOS 26 supported exactly the same devices as watchOS 11 before it: the Apple Watch Series 6 and later, Apple Watch SE (2nd generation) and later, and all Apple Watch Ultra models. With watchOS 27, Apple is effectively dropping three years' worth of device support in a single software update.
Update: Apple's initial compatibility list incorrectly said that the Apple Watch Series 9 would not support watchOS 27. We've updated this article following the company's confirmation that the device will indeed get the latest software.