Here is some single-subject data regarding how watchOS 8 on an S4 Apple Watch handles calorie counting for e-bike rides. I looked back at my Fitness app records and compared two rides that were almost the same. Both rides were on my e-bike using the lowest level of pedal assist through most of the ride. The first ride was recorded using watchOS 7 and the app reported that I was burning approximately 31 active cal per mile. The second ride was recorded using watchOS 8 and the app reported I was burning about 21 cal per mile. This suggest that watchOS 8 has changed in how calories are counted for outdoor cycling.
Next, I took my human-powered road bike out for the same route, tracking caloric effort with my Apple Watch running watchOS 8. It took more effort as demonstrated by the average heart rate of 107 BPM for this ride compared with 92 BPM for the same ride on my e-bike. My watch said I was burning 34 active cal per mile on my human-powered bike, much more than the 21 cal per mile reported for the same ride on my e-bike. I conclude from my data that the Apple Watch running watchOS 8 is making some under the hood adjustment for the reduced effort of riding an e-bike.
Attached are the Fitness app screens for the 3 rides. The 1st one is on my e-bike with watchOS 7, the 2nd one is my e-bike with watchOS 8, and the 3rd one is my human-powered bike with watchOS 8.