Apple Watch Series 8 double tap

No Double Tap is a series 9/Ultra 2 feature. You can use AssistiveTouch on the series 8 but that’s double tapping your side button

Use AssistiveTouch on your Apple Watch - Apple Support

No, only the Apple Watch Series 9 and Apple Watch Ultra 2 provide this functionality.

There is an article that explain how to simulate double-tap on non-supported models: https://appleinsider.com/inside/apple-watch/tips/how-to-get-double-tap-on-your-existing-apple-watch.