Currently, Apple WatchOS 26 can speak "split time" in workouts, but cannot announce notifications.
While iOS 26 can "Announce Notifications on Speaker" (if iPhone is locked" ), this is not available on Apple Watch
Please add this feature
Feature request: Apple Watch announce notifications
Thank you for your post and the enhancement request. I recommend filing the enhancement request through the appropriate channels, as the forums are not suitable for such submissions.
If you'd like us to consider adding the necessary functionality, please file an enhancement request using Feedback Assistant. Once you file the request, please post the FB number here.
If you're not familiar with how to file enhancement requests, take a look at Bug Reporting: How and Why?
Hope this helps.
Albert WWDR