Hi
The built-in Workout app offers an auto-pause setting for indoor and outdoor running workouts, but not for cycling workouts:
Work out with your Apple Watch - Apple Support
You may find that a third-party fitness tracking app meets your needs.
If you would like to send a feature request to Apple in respect of the Workout app, then you can do so here:
https://www.apple.com/feedback/watch.html
(You are not talking to Apple here - this is a user-based technical support community).