The Apple Watch Series 9 features a powerful new S9 chip that powers a number of new features that aren't available on older models. For example, the new double tap gesture is only supported on the Apple Watch Series 9 and Apple Watch Ultra 2, although you can get a very similar feature on older Apple Watches.
Original source: https://www.pocket-lint.com/how-to-use-siri-with-apple-health/
