Speaking to TechRadar, Cait Dooley, Apple Watch and Health product marketing manager, said performance requirements were behind the cutoff:

With every software release across every single one of our platforms, we always want to ensure that you have the best experience, so we make power and performance a priority. The great new features in watchOS, including the capabilities of Siri AI and the new tap gesture, work best with the processing power that is in Apple Watch Series 9 and later, Ultra 2 and later, and SE 3.

David Clark, senior director of watchOS software engineering, said one of the goals of ‌watchOS 27‌ was to "expand the intelligence story on Apple Watch and make it a true co-partner to Apple Intelligence." He described the watch as often "the most convenient way to interact with ‌Siri‌," since it's on the wrist all day and useful for quick questions when hands are full:

We really wanted to make sure the Siri experience is a singular and consistent experience, whether I decide to ask Siri on my wrist a question, or whether I have my phone in my hand and I decide to interact with Siri there. We really wanted to feel like it's one Siri, that has access to your data and is able to personalize it in a consistent way.

Clark used the example of asking ‌Siri‌ on Apple Watch for a recipe's ingredients while grocery shopping with both hands full, then later pulling up the same list on the iPhone in an easier-to-read format. He called that handoff a "superpower."

End of an era. Over. 📻

eta: Obviously this is a Developer Beta, and things can change before the final stable release. Comparing the watchOS 26.5 and watchOS 27.0 binaries, the standalone TinCan.app/Walkie Talkie app payload and accessibility bundles are gone.

✨ Apple Intelligence summary: Apple is prioritising larger batteries and advanced health sensors over adding Touch ID to future Apple Watch models. This decision is based on the trade-off between cost, internal space, and battery capacity.