I can say into my watch: Hey Siri, start Outdoor walk and it will open the Workout app on the watch and start that specific workout.
Later I say Hey Siri, stop workout and this used to work – the workout would stop.
Now Siri says I wish I could but workout is not set up with me and I cannot see any options to do that.
Best Answer
According to this Apple support page, it doesn't appear that Siri can end a workout.
The site only show Siri starting a workout, as you've already found. You can always submit feedback to Apple and suggest this feature be added.