According to TechCrunch, Apple Watch users will be able to delete stock apps from the Apple Watch starting in watchOS 6.
You’ll finally be able to delete Apple's built-in apps on the Apple Watch, including Alarm, Camera Remote, Radio, Walkie-Talkie, ECG, Breathe, Noise, and others. A few core apps like Heart Rate and Messages will not be deletable.
This functionality ins’t currently available in watchOS 6. It should be apart of the public release of the software update in September.