“Another tip we received is that Apple is already working on the next version of the watch that is expected to be released in 2021. While in Series 6, launched this year, the option of unlocking the watch will be added with a fingerprint embedded on the "crown" of the watch (the physical button on the side of the watch) in the next version ( Series 7?) Will support the fingerprint clock below the screen itself.”
The iPhone 8 series and every iPad bar the iPad Pro still has Touch ID, but the iPhone X-style iPhones now use Face ID, with no under display fingerprint sensors. Touch ID moving to the Watch makes sense given there is currently no way to biometrically identify yourself using only the Watch.
Putting Touch ID in the Digital Crown would mean it would be used for this function as well as the ECG feature, where electrodes sit to complete a circuit and take a heart reading on Series 4 and 5 of the Apple Watch. We reckon this is more likely than the under the glass option as Apple has not yet produced an under display sensor in any of its products.